diff --git a/src/Output/Checklist.php b/src/Output/Checklist.php
index da0e3759a..b586c499e 100644
--- a/src/Output/Checklist.php
+++ b/src/Output/Checklist.php
@@ -52,7 +52,7 @@ private function getLastItem(): mixed {
     return end($this->items);
   }
 
-  public function updateProgressBar(mixed $updateMessage): void {
+  public function updateProgressBar(string $updateMessage): void {
     $item = $this->getLastItem();
     if (!$item) {
       return;
diff --git a/tests/phpunit/src/Commands/ChecklistTest.php b/tests/phpunit/src/Commands/ChecklistTest.php
index 8150e13ae..c1c203449 100644
--- a/tests/phpunit/src/Commands/ChecklistTest.php
+++ b/tests/phpunit/src/Commands/ChecklistTest.php
@@ -28,7 +28,7 @@ public function testSpinner(): void {
     $checklist->addItem('Testing!');
 
     // Make the spinner spin with some output.
-    $outputCallback = static function (mixed $type, mixed $buffer) use ($checklist): void {
+    $outputCallback = static function (string $type, string $buffer) use ($checklist): void {
       $checklist->updateProgressBar($buffer);
     };
     $this->localMachineHelper->execute(['echo', 'hello world'], $outputCallback, NULL, FALSE);
@@ -39,6 +39,11 @@ public function testSpinner(): void {
     /** @var \Symfony\Component\Console\Helper\ProgressBar $progressBar */
     $progressBar = $items[0]['spinner']->getProgressBar();
     $this->assertEquals('Testing!', $progressBar->getMessage());
+    $this->assertEquals('<info>✔</info>', $progressBar->getBarCharacter());
+    $this->assertEquals('⢸', $progressBar->getProgressCharacter());
+    $this->assertEquals('⌛', $progressBar->getEmptyBarCharacter());
+    $this->assertEquals(1, $progressBar->getBarWidth());
+    $this->assertEquals('', $progressBar->getMessage('detail'));
 
     putenv('PHPUNIT_RUNNING');
   }