diff --git a/src/contracts/Collection/ArrayList.php b/src/contracts/Collection/ArrayList.php
index 738335b76a..706a83581d 100644
--- a/src/contracts/Collection/ArrayList.php
+++ b/src/contracts/Collection/ArrayList.php
@@ -27,7 +27,7 @@ public function __construct(array $items = [])
         parent::__construct(array_values($items));
     }
 
-    public function first()
+    public function first(): mixed
     {
         if (($result = reset($this->items)) !== false) {
             return $result;
@@ -36,7 +36,7 @@ public function first()
         throw new OutOfBoundsException('Collection is empty');
     }
 
-    public function last()
+    public function last(): mixed
     {
         if (($result = end($this->items)) !== false) {
             return $result;
diff --git a/src/contracts/Collection/ArrayMap.php b/src/contracts/Collection/ArrayMap.php
index 35c35f2b57..3632b2913f 100644
--- a/src/contracts/Collection/ArrayMap.php
+++ b/src/contracts/Collection/ArrayMap.php
@@ -21,7 +21,7 @@
  */
 class ArrayMap extends AbstractInMemoryCollection implements MapInterface
 {
-    public function get($key)
+    public function get($key): mixed
     {
         if (!$this->has($key)) {
             throw new OutOfBoundsException(sprintf("Collection does not contain element with key '%s'", $key));
diff --git a/src/contracts/Collection/ListInterface.php b/src/contracts/Collection/ListInterface.php
index 3ed8e5cf4c..bc1bff6ea0 100644
--- a/src/contracts/Collection/ListInterface.php
+++ b/src/contracts/Collection/ListInterface.php
@@ -22,7 +22,7 @@ interface ListInterface extends CollectionInterface
      *
      * @throws \Ibexa\Contracts\Core\Repository\Exceptions\OutOfBoundsException if collection is empty
      */
-    public function first();
+    public function first(): mixed;
 
     /**
      * Return last element of collection.
@@ -31,7 +31,7 @@ public function first();
      *
      * @throws \Ibexa\Contracts\Core\Repository\Exceptions\OutOfBoundsException if collection is empty
      */
-    public function last();
+    public function last(): mixed;
 
     /**
      * @param TValue $value
diff --git a/src/contracts/Collection/MapInterface.php b/src/contracts/Collection/MapInterface.php
index 8be64ed5fb..0026af4fb5 100644
--- a/src/contracts/Collection/MapInterface.php
+++ b/src/contracts/Collection/MapInterface.php
@@ -26,7 +26,7 @@ interface MapInterface extends CollectionInterface
      *
      * @throws \Ibexa\Contracts\Core\Repository\Exceptions\OutOfBoundsException if map does not contain element with given key
      */
-    public function get($key);
+    public function get($key): mixed;
 
     /**
      * Returns true if the given key is defined within the map.
diff --git a/src/contracts/Collection/MutableArrayList.php b/src/contracts/Collection/MutableArrayList.php
index a97b12d1e9..e477acbf15 100644
--- a/src/contracts/Collection/MutableArrayList.php
+++ b/src/contracts/Collection/MutableArrayList.php
@@ -17,17 +17,17 @@
  */
 class MutableArrayList extends ArrayList implements MutableListInterface
 {
-    public function append($value): void
+    public function append(mixed $value): void
     {
         $this->items[] = $value;
     }
 
-    public function prepend($value): void
+    public function prepend(mixed $value): void
     {
         array_unshift($this->items, $value);
     }
 
-    public function remove($value): void
+    public function remove(mixed $value): void
     {
         $idx = array_search($value, $this->items, true);
         if ($idx !== false) {
diff --git a/src/contracts/Collection/MutableListInterface.php b/src/contracts/Collection/MutableListInterface.php
index dfe4a38c5f..8256e26cfd 100644
--- a/src/contracts/Collection/MutableListInterface.php
+++ b/src/contracts/Collection/MutableListInterface.php
@@ -18,17 +18,17 @@ interface MutableListInterface extends ListInterface
     /**
      * @param TValue $value
      */
-    public function append($value): void;
+    public function append(mixed $value): void;
 
     /**
      * @param TValue $value
      */
-    public function prepend($value): void;
+    public function prepend(mixed $value): void;
 
     /**
      * @param TValue $value
      */
-    public function remove($value): void;
+    public function remove(mixed $value): void;
 
     public function clear(): void;
 }
diff --git a/src/contracts/Collection/MutableMapInterface.php b/src/contracts/Collection/MutableMapInterface.php
index 0805a66c83..2ebbd2eea4 100644
--- a/src/contracts/Collection/MutableMapInterface.php
+++ b/src/contracts/Collection/MutableMapInterface.php
@@ -20,12 +20,12 @@ interface MutableMapInterface extends MapInterface
      * @param TKey $key
      * @param TValue $value
      */
-    public function set($key, $value): void;
+    public function set(mixed $key, mixed $value): void;
 
     /**
      * @param TKey $key
      */
-    public function unset($key): void;
+    public function unset(mixed $key): void;
 
     public function clear(): void;
 }