diff --git a/src/Schema/ErrorCollection.php b/src/Schema/ErrorCollection.php
index ce948ac..5057a28 100644
--- a/src/Schema/ErrorCollection.php
+++ b/src/Schema/ErrorCollection.php
@@ -37,10 +37,20 @@ class ErrorCollection implements IteratorAggregate, ArrayAccess, Serializable, C
 {
     private array $items = [];
 
+    public function __serialize(): array
+    {
+        return ['items' => $this->items];
+    }
+
+    public function __unserialize(array $data): void
+    {
+        $this->items = $data['items'];
+    }
+
     /**
      * {@inheritdoc}
      */
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new ArrayIterator($this->items);
     }
@@ -48,7 +58,7 @@ public function getIterator()
     /**
      * {@inheritdoc}
      */
-    public function count()
+    public function count(): int
     {
         return \count($this->items);
     }
@@ -72,7 +82,7 @@ public function unserialize($serialized)
     /**
      * {@inheritdoc}
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return isset($this->items[$offset]);
     }
@@ -82,6 +92,7 @@ public function offsetExists($offset)
      *
      * @return ErrorInterface
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->items[$offset];
@@ -90,7 +101,8 @@ public function offsetGet($offset)
     /**
      * {@inheritdoc}
      */
-    public function offsetSet($offset, $value)
+    #[\ReturnTypeWillChange]
+    public function offsetSet($offset, $value): void
     {
         null === $offset ? $this->add($value) : $this->items[$offset] = $value;
     }
@@ -98,7 +110,8 @@ public function offsetSet($offset, $value)
     /**
      * {@inheritdoc}
      */
-    public function offsetUnset($offset)
+    #[\ReturnTypeWillChange]
+    public function offsetUnset($offset): void
     {
         unset($this->items[$offset]);
     }
diff --git a/tests/Data/Collection.php b/tests/Data/Collection.php
index fe8d878..0642406 100644
--- a/tests/Data/Collection.php
+++ b/tests/Data/Collection.php
@@ -31,14 +31,12 @@ class Collection implements ArrayAccess, IteratorAggregate
     /**
      * {@inheritdoc}
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->data);
     }
 
-    /**
-     * {@inheritdoc}
-     */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->data[$offset];
@@ -47,7 +45,7 @@ public function offsetGet($offset)
     /**
      * {@inheritdoc}
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         null === $offset ? $this->data[] = $value : $this->data[$offset] = $value;
     }
@@ -55,7 +53,7 @@ public function offsetSet($offset, $value)
     /**
      * {@inheritdoc}
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->data[$offset]);
     }
@@ -63,7 +61,7 @@ public function offsetUnset($offset)
     /**
      * {@inheritdoc}
      */
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new ArrayIterator($this->data);
     }
diff --git a/tests/Data/Models/AuthorCModel.php b/tests/Data/Models/AuthorCModel.php
index b04917f..cecfd41 100644
--- a/tests/Data/Models/AuthorCModel.php
+++ b/tests/Data/Models/AuthorCModel.php
@@ -50,7 +50,7 @@ public function __construct(int $identity, string $firstName, string $lastName,
     /**
      * {@inheritdoc}
      */
-    public function getIterator()
+    public function getIterator(): \Traversable
     {
         return new ArrayIterator($this->properties);
     }
@@ -58,7 +58,7 @@ public function getIterator()
     /**
      * {@inheritdoc}
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return \array_key_exists($offset, $this->properties);
     }
@@ -66,6 +66,7 @@ public function offsetExists($offset)
     /**
      * {@inheritdoc}
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
         return $this->properties[$offset];
@@ -74,7 +75,7 @@ public function offsetGet($offset)
     /**
      * {@inheritdoc}
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         $this->properties[$offset] = $value;
     }
@@ -82,7 +83,7 @@ public function offsetSet($offset, $value)
     /**
      * {@inheritdoc}
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->properties[$offset]);
     }