diff --git a/Models/KanbanBoard.php b/Models/KanbanBoard.php
index 22168a3..ddc9c86 100755
--- a/Models/KanbanBoard.php
+++ b/Models/KanbanBoard.php
@@ -268,7 +268,7 @@ class KanbanBoard implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/KanbanCard.php b/Models/KanbanCard.php
index c1e7ac5..302df3e 100755
--- a/Models/KanbanCard.php
+++ b/Models/KanbanCard.php
@@ -383,7 +383,7 @@ class KanbanCard implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/KanbanCardComment.php b/Models/KanbanCardComment.php
index c7566c6..70c1b86 100755
--- a/Models/KanbanCardComment.php
+++ b/Models/KanbanCardComment.php
@@ -151,7 +151,7 @@ class KanbanCardComment implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/KanbanColumn.php b/Models/KanbanColumn.php
index c7395e2..4e9edbd 100755
--- a/Models/KanbanColumn.php
+++ b/Models/KanbanColumn.php
@@ -139,7 +139,7 @@ class KanbanColumn implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }