diff --git a/Models/Employee.php b/Models/Employee.php
index ade8615..92f1f3a 100755
--- a/Models/Employee.php
+++ b/Models/Employee.php
@@ -295,7 +295,7 @@ class Employee implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/EmployeeEducationHistory.php b/Models/EmployeeEducationHistory.php
index e9ac51d..3f6dd14 100755
--- a/Models/EmployeeEducationHistory.php
+++ b/Models/EmployeeEducationHistory.php
@@ -121,7 +121,7 @@ class EmployeeEducationHistory implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/EmployeeHistory.php b/Models/EmployeeHistory.php
index bfff468..17a8053 100755
--- a/Models/EmployeeHistory.php
+++ b/Models/EmployeeHistory.php
@@ -144,7 +144,7 @@ class EmployeeHistory implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/EmployeeWorkHistory.php b/Models/EmployeeWorkHistory.php
index 7115a10..6080fb7 100755
--- a/Models/EmployeeWorkHistory.php
+++ b/Models/EmployeeWorkHistory.php
@@ -115,7 +115,7 @@ class EmployeeWorkHistory implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }