diff --git a/composer.json b/composer.json
index 588e34e..87159a7 100644
--- a/composer.json
+++ b/composer.json
@@ -21,8 +21,8 @@
         }
     ],
     "require": {
-        "php": ">=5.6.0",
-        "league/fractal": "~0.17"
+        "php": ">=7.4",
+        "league/fractal": "~0.20"
     },
     "require-dev": {
         "phpunit/phpunit": "~5.0",
diff --git a/src/Serializer/Serializer.php b/src/Serializer/Serializer.php
index c9035b9..690e087 100644
--- a/src/Serializer/Serializer.php
+++ b/src/Serializer/Serializer.php
@@ -21,7 +21,7 @@ class Serializer extends ArraySerializer
      *
      * @return array
      */
-    public function collection($resourceKey, array $data)
+    public function collection(string $resourceKey, array $data): array
     {
         if (is_null($resourceKey)) $resourceKey = static::RESOURCE_KEY;
         return $resourceKey ? [$resourceKey => $data]: $data;
@@ -35,7 +35,7 @@ public function collection($resourceKey, array $data)
      *
      * @return array
      */
-    public function item($resourceKey, array $data)
+    public function item(string $resourceKey, array $data): array
     {
         if (is_null($resourceKey)) $resourceKey = static::RESOURCE_KEY;
         return $resourceKey ? [$resourceKey => $data]: $data;