diff --git a/Build/Scripts/phpstan.sh b/Build/Scripts/phpstan.sh
new file mode 100755
index 0000000..7a95b00
--- /dev/null
+++ b/Build/Scripts/phpstan.sh
@@ -0,0 +1,13 @@
+#!/bin/bash
+
+THIS_SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" >/dev/null && pwd)"
+cd "$THIS_SCRIPT_DIR" || exit 1
+cd ../../ || exit 1
+CORE_ROOT="${PWD}"
+
+Build/Scripts/runTests.sh -s composerInstall
+
+Build/Scripts/runTests.sh -s phpstan
+
+Build/Scripts/runTests.sh -s clean
+Build/Scripts/additionalTests.sh -s clean
diff --git a/Build/phpstan/phpstan-baseline.neon b/Build/phpstan/phpstan-baseline.neon
new file mode 100644
index 0000000..1339890
--- /dev/null
+++ b/Build/phpstan/phpstan-baseline.neon
@@ -0,0 +1,2 @@
+parameters:
+	ignoreErrors:
\ No newline at end of file
diff --git a/Build/phpstan/phpstan-constants.php b/Build/phpstan/phpstan-constants.php
new file mode 100644
index 0000000..0bb0280
--- /dev/null
+++ b/Build/phpstan/phpstan-constants.php
@@ -0,0 +1,4 @@
+<?php
+
+// testing-framework defines this, used in various tests.
+define('ORIGINAL_ROOT', dirname(__FILE__, 2) . '/');
diff --git a/Build/phpstan/phpstan.local.neon b/Build/phpstan/phpstan.local.neon
new file mode 100644
index 0000000..d89a1f1
--- /dev/null
+++ b/Build/phpstan/phpstan.local.neon
@@ -0,0 +1,2 @@
+includes:
+  - phpstan.neon
diff --git a/Build/phpstan/phpstan.neon b/Build/phpstan/phpstan.neon
new file mode 100644
index 0000000..e7d5a93
--- /dev/null
+++ b/Build/phpstan/phpstan.neon
@@ -0,0 +1,21 @@
+includes:
+  - phpstan-baseline.neon
+  - ../vendor/friendsoftypo3/phpstan-typo3/extension.neon
+
+parameters:
+  # Use local .cache dir instead of /tmp
+  tmpDir: ../.cache/phpstan
+
+  level: 6
+
+  bootstrapFiles:
+    - phpstan-constants.php
+
+  paths:
+    - ../../
+
+  excludePaths:
+    # we do not check required extensions
+    - ../../Build/*
+    # ext_emconf.php get the $_EXTKEY set from outside. We'll ignore all of them
+    - ../ext_emconf.php
diff --git a/Classes/Controller/AbstractController.php b/Classes/Controller/AbstractController.php
index 577af59..dba24c5 100644
--- a/Classes/Controller/AbstractController.php
+++ b/Classes/Controller/AbstractController.php
@@ -15,6 +15,14 @@
 
 namespace Evoweb\SfBooks\Controller;
 
+use Evoweb\SfBooks\Domain\Model\Author;
+use Evoweb\SfBooks\Domain\Model\Book;
+use Evoweb\SfBooks\Domain\Model\Category;
+use Evoweb\SfBooks\Domain\Model\Series;
+use Evoweb\SfBooks\Domain\Repository\AuthorRepository;
+use Evoweb\SfBooks\Domain\Repository\BookRepository;
+use Evoweb\SfBooks\Domain\Repository\CategoryRepository;
+use Evoweb\SfBooks\Domain\Repository\SeriesRepository;
 use Evoweb\SfBooks\TitleTagProvider\TitleTagProvider;
 use TYPO3\CMS\Core\Core\Environment;
 use TYPO3\CMS\Core\Http\ImmediateResponseException;
@@ -24,18 +32,19 @@
 use TYPO3\CMS\Core\Pagination\SimplePagination;
 use TYPO3\CMS\Core\Resource\ResourceFactory;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
+use TYPO3\CMS\Extbase\DomainObject\AbstractEntity;
 use TYPO3\CMS\Extbase\Mvc\Controller\ActionController;
 use TYPO3\CMS\Extbase\Pagination\QueryResultPaginator;
 use TYPO3\CMS\Extbase\Persistence\QueryInterface;
 use TYPO3\CMS\Extbase\Persistence\QueryResultInterface;
-use TYPO3\CMS\Extbase\Persistence\Repository;
 use TYPO3\CMS\Frontend\Controller\ErrorController;
 use TYPO3Fluid\Fluid\View\ViewInterface;
 
 abstract class AbstractController extends ActionController
 {
-    protected function setDefaultOrderings(Repository $repository): Repository
-    {
+    protected function setDefaultOrderings(
+        AuthorRepository|BookRepository|CategoryRepository|SeriesRepository $repository
+    ): AuthorRepository|BookRepository|CategoryRepository|SeriesRepository {
         $orderField = $this->getOrderField();
         if ($orderField !== '') {
             $orderings = [$orderField => $this->getOrderDirection()];
@@ -116,6 +125,9 @@ protected function displayError(string $type): void
         throw new ImmediateResponseException($response);
     }
 
+    /**
+     * @param QueryResultInterface<int, Author>|QueryResultInterface<int, Book>|QueryResultInterface<int, Category>|array<int, Series> $result
+     */
     protected function addPaginatorToView(QueryResultInterface|array $result): void
     {
         $paginator = $this->getPaginator($result);
@@ -130,6 +142,9 @@ protected function addPaginatorToView(QueryResultInterface|array $result): void
         );
     }
 
+    /**
+     * @param QueryResultInterface<int, AbstractEntity>|array<int, AbstractEntity> $result
+     */
     protected function getPaginator(QueryResultInterface|array $result): PaginatorInterface
     {
         $currentPage = $this->request->hasArgument('currentPage')
diff --git a/Classes/Controller/CategoryController.php b/Classes/Controller/CategoryController.php
index ed1d603..9a4ee5f 100644
--- a/Classes/Controller/CategoryController.php
+++ b/Classes/Controller/CategoryController.php
@@ -57,6 +57,10 @@ protected function listAction(): ResponseInterface
         return new HtmlResponse($this->view->render());
     }
 
+    /**
+     * @param QueryResultInterface<int, Category> $categories
+     * @return QueryResultInterface<int, Category>
+     */
     protected function removeExcludeCategories(QueryResultInterface $categories): QueryResultInterface
     {
         $excludeCategories = GeneralUtility::intExplode(',', $this->settings['excludeCategories'], true);
diff --git a/Classes/Controller/SearchController.php b/Classes/Controller/SearchController.php
index 3a7c03e..71c0ec6 100644
--- a/Classes/Controller/SearchController.php
+++ b/Classes/Controller/SearchController.php
@@ -28,11 +28,14 @@ public function searchAction(): ResponseInterface
         return new HtmlResponse($this->view->render());
     }
 
+    /**
+     * @param array<string, mixed> $search
+     */
     public function startSearchAction(array $search): ResponseInterface
     {
         if (($search['query'] ?? '') != '') {
             if (isset($search['searchBy'])) {
-                switch ((string)$search['searchFor'] ?? '') {
+                switch ((string)($search['searchFor'] ?? '')) {
                     case 'author':
                         $controller = 'Author';
                         $pageId = (int)$this->settings['authorPageId'];
@@ -49,7 +52,7 @@ public function startSearchAction(array $search): ResponseInterface
                     $pageId = $this->request->getAttribute('currentContentObject')->data['pid'];
                 }
 
-                $this->redirect('search', $controller, null, $search, $pageId, $controller);
+                return $this->redirect('search', $controller, null, $search, $pageId);
             }
             $response = new HtmlResponse($this->view->render());
         } else {
@@ -59,6 +62,9 @@ public function startSearchAction(array $search): ResponseInterface
         return $response;
     }
 
+    /**
+     * @param ?array<string, mixed> $arguments
+     */
     protected function redirect(
         $actionName,
         $controllerName = null,
diff --git a/Classes/Domain/Model/Author.php b/Classes/Domain/Model/Author.php
index 993ad92..8e2a667 100644
--- a/Classes/Domain/Model/Author.php
+++ b/Classes/Domain/Model/Author.php
@@ -45,11 +45,17 @@ public function initializeObject(): void
         $this->books = new ObjectStorage();
     }
 
+    /**
+     * @param ObjectStorage<Book> $books
+     */
     public function setBooks(ObjectStorage $books): void
     {
         $this->books = $books;
     }
 
+    /**
+     * @return ObjectStorage<Book>
+     */
     public function getBooks(): ObjectStorage
     {
         return $this->books;
diff --git a/Classes/Domain/Model/Book.php b/Classes/Domain/Model/Book.php
index 19624f0..9366788 100644
--- a/Classes/Domain/Model/Book.php
+++ b/Classes/Domain/Model/Book.php
@@ -91,61 +91,97 @@ public function initializeObject(): void
         $this->samplePdf = new ObjectStorage();
     }
 
+    /**
+     * @param ObjectStorage<Author> $author
+     */
     public function setAuthor(ObjectStorage $author): void
     {
         $this->author = $author;
     }
 
+    /**
+     * @return ObjectStorage<Author>
+     */
     public function getAuthor(): ObjectStorage
     {
         return $this->author;
     }
 
+    /**
+     * @param ObjectStorage<Category> $category
+     */
     public function setCategory(ObjectStorage $category): void
     {
         $this->category = $category;
     }
 
+    /**
+     * @return ObjectStorage<Category>
+     */
     public function getCategory(): ObjectStorage
     {
         return $this->category;
     }
 
+    /**
+     * @param ObjectStorage<Extras> $extras
+     */
     public function setExtras(ObjectStorage $extras): void
     {
         $this->extras = $extras;
     }
 
+    /**
+     * @return ObjectStorage<Extras>
+     */
     public function getExtras(): ObjectStorage
     {
         return $this->extras;
     }
 
+    /**
+     * @param ObjectStorage<FileReference> $cover
+     */
     public function setCover(ObjectStorage $cover): void
     {
         $this->cover = $cover;
     }
 
+    /**
+     * @return ObjectStorage<FileReference>
+     */
     public function getCover(): ObjectStorage
     {
         return $this->cover;
     }
 
+    /**
+     * @param ObjectStorage<FileReference> $coverLarge
+     */
     public function setCoverLarge(ObjectStorage $coverLarge): void
     {
         $this->coverLarge = $coverLarge;
     }
 
+    /**
+     * @return ObjectStorage<FileReference>
+     */
     public function getCoverLarge(): ObjectStorage
     {
         return $this->coverLarge;
     }
 
+    /**
+     * @param ObjectStorage<FileReference> $samplePdf
+     */
     public function setSamplePdf(ObjectStorage $samplePdf): void
     {
         $this->samplePdf = $samplePdf;
     }
 
+    /**
+     * @return ObjectStorage<FileReference>
+     */
     public function getSamplePdf(): ObjectStorage
     {
         return $this->samplePdf;
diff --git a/Classes/Domain/Model/Category.php b/Classes/Domain/Model/Category.php
index 7cff72e..e431f17 100644
--- a/Classes/Domain/Model/Category.php
+++ b/Classes/Domain/Model/Category.php
@@ -53,21 +53,33 @@ public function initializeObject(): void
         $this->books = new ObjectStorage();
     }
 
+    /**
+     * @param ObjectStorage<Category> $children
+     */
     public function setChildren(ObjectStorage $children): void
     {
         $this->children = $children;
     }
 
+    /**
+     * @return ObjectStorage<Category>
+     */
     public function getChildren(): ObjectStorage
     {
         return $this->children;
     }
 
+    /**
+     * @param ObjectStorage<Book> $books
+     */
     public function setBooks(ObjectStorage $books): void
     {
         $this->books = $books;
     }
 
+    /**
+     * @return ObjectStorage<Book>
+     */
     public function getBooks(): ObjectStorage
     {
         return $this->books;
diff --git a/Classes/Domain/Model/Series.php b/Classes/Domain/Model/Series.php
index 8240850..d5dc0b7 100644
--- a/Classes/Domain/Model/Series.php
+++ b/Classes/Domain/Model/Series.php
@@ -46,11 +46,17 @@ public function initializeObject(): void
         $this->books = new ObjectStorage();
     }
 
+    /**
+     * @param ObjectStorage<Book> $books
+     */
     public function setBooks(ObjectStorage $books): void
     {
         $this->books = $books;
     }
 
+    /**
+     * @return ObjectStorage<Book>
+     */
     public function getBooks(): ObjectStorage
     {
         return $this->books;
diff --git a/Classes/Domain/Repository/AuthorRepository.php b/Classes/Domain/Repository/AuthorRepository.php
index 42841f5..43e8622 100644
--- a/Classes/Domain/Repository/AuthorRepository.php
+++ b/Classes/Domain/Repository/AuthorRepository.php
@@ -19,10 +19,18 @@
 use TYPO3\CMS\Core\Database\ConnectionPool;
 use TYPO3\CMS\Core\Database\Query\QueryBuilder;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
+use TYPO3\CMS\Extbase\Persistence\Exception\InvalidQueryException;
+use TYPO3\CMS\Extbase\Persistence\Generic\Exception;
+use TYPO3\CMS\Extbase\Persistence\Generic\Query;
 use TYPO3\CMS\Extbase\Persistence\PersistenceManagerInterface;
 use TYPO3\CMS\Extbase\Persistence\QueryResultInterface;
 use TYPO3\CMS\Extbase\Persistence\Repository;
 
+/**
+ * A repository for authors
+ *
+ * @extends Repository<Author>
+ */
 class AuthorRepository extends Repository
 {
     public function __construct(
@@ -33,8 +41,13 @@ public function __construct(
         parent::__construct();
     }
 
+    /**
+     * @return array<string, mixed>
+     * @throws Exception
+     */
     public function findAuthorGroupedByLetters(): array
     {
+        /** @var Query<Author> $query */
         $query = $this->createQuery();
 
         $queryBuilder = $this->getQueryBuilderForTable('tx_sfbooks_domain_model_author');
@@ -57,7 +70,7 @@ public function findAuthorGroupedByLetters(): array
         /** @var Author $author */
         foreach ($result as $author) {
             $letter = $author->getCapitalLetter();
-            if (!isset($groupedAuthors[$letter]) || !is_array($groupedAuthors[$letter])) {
+            if (!is_array($groupedAuthors[$letter] ?? '')) {
                 $groupedAuthors[$letter] = [];
             }
 
@@ -67,6 +80,11 @@ public function findAuthorGroupedByLetters(): array
         return $groupedAuthors;
     }
 
+    /**
+     * @param string[] $searchFields
+     * @return QueryResultInterface<int, Author>
+     * @throws InvalidQueryException
+     */
     public function findBySearch(string $searchString, array $searchFields): QueryResultInterface
     {
         $query = $this->createQuery();
diff --git a/Classes/Domain/Repository/BookRepository.php b/Classes/Domain/Repository/BookRepository.php
index 2b630cf..af06e87 100644
--- a/Classes/Domain/Repository/BookRepository.php
+++ b/Classes/Domain/Repository/BookRepository.php
@@ -15,11 +15,23 @@
 
 namespace Evoweb\SfBooks\Domain\Repository;
 
+use Evoweb\SfBooks\Domain\Model\Book;
+use TYPO3\CMS\Extbase\Persistence\Exception\InvalidQueryException;
 use TYPO3\CMS\Extbase\Persistence\QueryResultInterface;
 use TYPO3\CMS\Extbase\Persistence\Repository;
 
+/**
+ * A repository for books
+ *
+ * @extends Repository<Book>
+ */
 class BookRepository extends Repository
 {
+    /**
+     * @param int[] $categories
+     * @return QueryResultInterface<int, Book>
+     * @throws InvalidQueryException
+     */
     public function findByCategories(array $categories): QueryResultInterface
     {
         $query = $this->createQuery();
@@ -35,6 +47,11 @@ public function findByCategories(array $categories): QueryResultInterface
         return $query->execute();
     }
 
+    /**
+     * @param string[] $searchFields
+     * @return QueryResultInterface<int, Book>
+     * @throws InvalidQueryException
+     */
     public function findBySearch(string $searchString, array $searchFields): QueryResultInterface
     {
         $query = $this->createQuery();
diff --git a/Classes/Domain/Repository/CategoryRepository.php b/Classes/Domain/Repository/CategoryRepository.php
index 3611bc2..d8cf118 100644
--- a/Classes/Domain/Repository/CategoryRepository.php
+++ b/Classes/Domain/Repository/CategoryRepository.php
@@ -15,11 +15,23 @@
 
 namespace Evoweb\SfBooks\Domain\Repository;
 
+use Evoweb\SfBooks\Domain\Model\Category;
+use TYPO3\CMS\Extbase\Persistence\Exception\InvalidQueryException;
 use TYPO3\CMS\Extbase\Persistence\QueryResultInterface;
 use TYPO3\CMS\Extbase\Persistence\Repository;
 
+/**
+ * A repository for categories
+ *
+ * @extends Repository<Category>
+ */
 class CategoryRepository extends Repository
 {
+    /**
+     * @param int[] $categories
+     * @return QueryResultInterface<int, Category>
+     * @throws InvalidQueryException
+     */
     public function findByUids(array $categories): QueryResultInterface
     {
         $query = $this->createQuery();
diff --git a/Classes/Domain/Repository/SeriesRepository.php b/Classes/Domain/Repository/SeriesRepository.php
index 27c53ea..b4d67e6 100644
--- a/Classes/Domain/Repository/SeriesRepository.php
+++ b/Classes/Domain/Repository/SeriesRepository.php
@@ -18,10 +18,17 @@
 use Evoweb\SfBooks\Domain\Model\Series;
 use TYPO3\CMS\Core\Database\ConnectionPool;
 use TYPO3\CMS\Core\Database\Query\QueryBuilder;
+use TYPO3\CMS\Extbase\Persistence\Generic\Exception;
+use TYPO3\CMS\Extbase\Persistence\Generic\Query;
 use TYPO3\CMS\Extbase\Persistence\PersistenceManagerInterface;
 use TYPO3\CMS\Extbase\Persistence\QueryResultInterface;
 use TYPO3\CMS\Extbase\Persistence\Repository;
 
+/**
+ * A repository for series
+ *
+ * @extends Repository<Series>
+ */
 class SeriesRepository extends Repository
 {
     public function __construct(
@@ -32,8 +39,13 @@ public function __construct(
         parent::__construct();
     }
 
+    /**
+     * @return array<string, array<Series>>
+     * @throws Exception
+     */
     public function findSeriesGroupedByLetters(): array
     {
+        /** @var Query<Series> $query */
         $query = $this->createQuery();
 
         $queryBuilder = $this->getQueryBuilderForTable('tx_sfbooks_domain_model_series');
@@ -56,7 +68,7 @@ public function findSeriesGroupedByLetters(): array
         /** @var Series $series */
         foreach ($result as $series) {
             $letter = $series->getCapitalLetter();
-            if (!isset($groupedSeries[$letter]) || !is_array($groupedSeries[$letter])) {
+            if (!is_array($groupedSeries[$letter] ?? '')) {
                 $groupedSeries[$letter] = [];
             }
 
@@ -66,6 +78,10 @@ public function findSeriesGroupedByLetters(): array
         return $groupedSeries;
     }
 
+    /**
+     * @param int[] $series
+     * @return QueryResultInterface<int, Series>
+     */
     public function findBySeries(array $series): QueryResultInterface
     {
         $query = $this->createQuery();
diff --git a/Classes/User/IsbnEvaluation.php b/Classes/User/IsbnEvaluation.php
index 3830065..3dd0941 100644
--- a/Classes/User/IsbnEvaluation.php
+++ b/Classes/User/IsbnEvaluation.php
@@ -34,22 +34,24 @@ public function returnFieldJS(): JavaScriptModuleInstruction
     /**
      * Server-side validation/evaluation on saving the record
      *
-     * @param array $parameters The parameters value, is_in and set
+     * @param string[] $parameters The parameters value, is_in and set
      * @return string Evaluated field value
      */
     public function evaluateFieldValue(...$parameters): string
     {
-        return preg_replace('/[^0-9X\-]/i', '', $parameters[0]);
+        $parameter = $parameters[0] ?? '';
+        return preg_replace('/[^0-9X\-]/i', '', $parameter);
     }
 
     /**
      * Server-side validation/evaluation on opening the record
      *
-     * @param array $parameters Array with key 'value' containing the field value from the database
+     * @param array<string, string> $parameters Array with key 'value' containing the field value from the database
      * @return string Evaluated field value
      */
     public function deevaluateFieldValue(array $parameters): string
     {
-        return preg_replace('/[^0-9X\-]/i', '', $parameters['value']);
+        $value = $parameters['value'] ?? '';
+        return preg_replace('/[^0-9X\-]/i', '', $value);
     }
 }
diff --git a/Tests/Functional/AbstractTestCase.php b/Tests/Functional/AbstractTestCase.php
index 98d54fd..be928e6 100644
--- a/Tests/Functional/AbstractTestCase.php
+++ b/Tests/Functional/AbstractTestCase.php
@@ -13,12 +13,12 @@
 
 namespace Evoweb\SfBooks\Tests\Functional;
 
+use Doctrine\DBAL\Exception;
 use Psr\Http\Message\ServerRequestInterface;
 use TYPO3\CMS\Core\Core\SystemEnvironmentBuilder;
 use TYPO3\CMS\Core\Database\ConnectionPool;
 use TYPO3\CMS\Core\Http\NormalizedParams;
 use TYPO3\CMS\Core\Http\ServerRequest;
-use TYPO3\CMS\Core\TypoScript\FrontendTypoScript;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
 use TYPO3\TestingFramework\Core\Functional\FunctionalTestCase;
 
@@ -40,7 +40,7 @@ protected function setUp(): void
         $GLOBALS['TYPO3_REQUEST'] = $request;
     }
 
-    private function createServerRequest(string $url, string $method = 'GET'): ServerRequestInterface
+    private function createServerRequest(string $url): ServerRequestInterface
     {
         $requestUrlParts = parse_url($url);
         $docRoot = $this->instancePath;
@@ -58,7 +58,7 @@ private function createServerRequest(string $url, string $method = 'GET'): Serve
             'PATH_TRANSLATED' => $docRoot . '/index.php',
             'QUERY_STRING' => $requestUrlParts['query'] ?? '',
             'REQUEST_URI' => $requestUrlParts['path'] . (isset($requestUrlParts['query']) ? '?' . $requestUrlParts['query'] : ''),
-            'REQUEST_METHOD' => $method,
+            'REQUEST_METHOD' => 'GET',
         ];
         // Define HTTPS and server port
         if (isset($requestUrlParts['scheme'])) {
@@ -75,7 +75,7 @@ private function createServerRequest(string $url, string $method = 'GET'): Serve
             $serverParams['SERVER_PORT'] = $requestUrlParts['port'];
         }
         // set up normalizedParams
-        $request = new ServerRequest($url, $method, null, [], $serverParams);
+        $request = new ServerRequest($url, 'GET', null, [], $serverParams);
         $request = $request->withAttribute('applicationType', SystemEnvironmentBuilder::REQUESTTYPE_BE);
         return $request->withAttribute('normalizedParams', NormalizedParams::createFromRequest($request));
     }
@@ -101,9 +101,13 @@ protected function assertNoLogEntries(): void
         }
     }
 
+    /**
+     * @return array<array<string, mixed>>
+     */
     protected function getLogEntries(): array
     {
-        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)->getQueryBuilderForTable('sys_log');
+        $queryBuilder = GeneralUtility::makeInstance(ConnectionPool::class)
+            ->getQueryBuilderForTable('sys_log');
         $result = $queryBuilder
             ->select('*')
             ->from('sys_log')
@@ -113,8 +117,11 @@ protected function getLogEntries(): array
                     [1, 2]
                 )
             )
-            ->executeQuery()
-            ->fetchAllAssociative();
+            ->executeQuery();
+        try {
+            $result = $result->fetchAllAssociative();
+        } catch (Exception) {
+        }
         return is_array($result) ? $result : [];
     }
 }
diff --git a/Tests/Functional/Domain/Repository/AuthorRepositoryTest.php b/Tests/Functional/Domain/Repository/AuthorRepositoryTest.php
index 7853013..a03d4ce 100644
--- a/Tests/Functional/Domain/Repository/AuthorRepositoryTest.php
+++ b/Tests/Functional/Domain/Repository/AuthorRepositoryTest.php
@@ -37,7 +37,7 @@ protected function setUp(): void
     }
 
     #[Test]
-    public function findByUidReturnsOneAuthor()
+    public function findByUidReturnsOneAuthor(): void
     {
         $author = $this->subject->findByUid(1);
         $properties = [
@@ -62,7 +62,7 @@ public function findByUidReturnsOneAuthor()
     }
 
     #[Test]
-    public function findAuthorGroupedByLetters()
+    public function findAuthorGroupedByLetters(): void
     {
         $result = $this->subject->findAuthorGroupedByLetters();
         /** @var Author $author */
diff --git a/Tests/Functional/Domain/Repository/SeriesRepositoryTest.php b/Tests/Functional/Domain/Repository/SeriesRepositoryTest.php
index d0104ce..f297671 100644
--- a/Tests/Functional/Domain/Repository/SeriesRepositoryTest.php
+++ b/Tests/Functional/Domain/Repository/SeriesRepositoryTest.php
@@ -37,7 +37,7 @@ protected function setUp(): void
     }
 
     #[Test]
-    public function findByUidReturnsOneSeries()
+    public function findByUidReturnsOneSeries(): void
     {
         /** @var Series $series */
         $series = $this->subject->findByUid(1);
@@ -63,7 +63,7 @@ public function findByUidReturnsOneSeries()
     }
 
     #[Test]
-    public function findSeriesGroupedByLetters()
+    public function findSeriesGroupedByLetters(): void
     {
         $result = $this->subject->findSeriesGroupedByLetters();
         /** @var Series $series */
diff --git a/composer.json b/composer.json
index 2488ebf..7cfcfd4 100644
--- a/composer.json
+++ b/composer.json
@@ -32,10 +32,12 @@
 		"doctrine/dbal": "^3.7.2 || ^4.0"
 	},
 	"require-dev": {
-		"friendsofphp/php-cs-fixer": "^3.57.1",
-		"typo3/testing-framework": "dev-main",
-		"phpunit/phpunit": "^11.3.1",
-		"webmozart/assert": "^1.11.0"
+		"friendsofphp/php-cs-fixer": "^3.64.0",
+		"friendsoftypo3/phpstan-typo3": "^0.9.0",
+		"phpstan/phpdoc-parser": "^1.30.0",
+		"phpstan/phpstan": "^1.12.5",
+		"phpunit/phpunit": "^11.0.3",
+		"typo3/testing-framework": "dev-main"
 	},
 	"minimum-stability": "dev",
 	"prefer-stable": true,