diff --git a/Models/Department.php b/Models/Department.php
index a2f727d..c8fc870 100755
--- a/Models/Department.php
+++ b/Models/Department.php
@@ -153,7 +153,7 @@ class Department implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/Position.php b/Models/Position.php
index d6154d6..c251027 100755
--- a/Models/Position.php
+++ b/Models/Position.php
@@ -150,7 +150,7 @@ class Position implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/Unit.php b/Models/Unit.php
index 966c8e0..c387d46 100755
--- a/Models/Unit.php
+++ b/Models/Unit.php
@@ -152,7 +152,7 @@ class Unit implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }