diff --git a/Models/AccountRelation.php b/Models/AccountRelation.php
index 3c2d586..e3b7d97 100755
--- a/Models/AccountRelation.php
+++ b/Models/AccountRelation.php
@@ -76,7 +76,7 @@ class AccountRelation extends RelationAbstract
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/GroupRelation.php b/Models/GroupRelation.php
index 6626186..f099571 100755
--- a/Models/GroupRelation.php
+++ b/Models/GroupRelation.php
@@ -76,7 +76,7 @@ class GroupRelation extends RelationAbstract
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/Task.php b/Models/Task.php
index fc165b6..f8c3630 100755
--- a/Models/Task.php
+++ b/Models/Task.php
@@ -616,7 +616,7 @@ class Task implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/TaskElement.php b/Models/TaskElement.php
index c650f1d..87a0c4f 100755
--- a/Models/TaskElement.php
+++ b/Models/TaskElement.php
@@ -535,7 +535,7 @@ class TaskElement implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }