diff --git a/Models/Project.php b/Models/Project.php
index accb4b9..c5561b0 100755
--- a/Models/Project.php
+++ b/Models/Project.php
@@ -434,7 +434,7 @@ class Project
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/ProjectAttribute.php b/Models/ProjectAttribute.php
index 3c0196e..9222c1b 100755
--- a/Models/ProjectAttribute.php
+++ b/Models/ProjectAttribute.php
@@ -97,7 +97,7 @@ class ProjectAttribute implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/ProjectAttributeType.php b/Models/ProjectAttributeType.php
index a5a09ab..77a7908 100755
--- a/Models/ProjectAttributeType.php
+++ b/Models/ProjectAttributeType.php
@@ -165,7 +165,7 @@ class ProjectAttributeType implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/ProjectAttributeTypeL11n.php b/Models/ProjectAttributeTypeL11n.php
index 1bc5d8a..9aa2072 100755
--- a/Models/ProjectAttributeTypeL11n.php
+++ b/Models/ProjectAttributeTypeL11n.php
@@ -129,7 +129,7 @@ class ProjectAttributeTypeL11n implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/ProjectAttributeValue.php b/Models/ProjectAttributeValue.php
index cccaee3..809330d 100755
--- a/Models/ProjectAttributeValue.php
+++ b/Models/ProjectAttributeValue.php
@@ -248,7 +248,7 @@ class ProjectAttributeValue implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }