diff --git a/Interfaces/GSD/Model/GSDCostCenter.php b/Interfaces/GSD/Model/GSDCostCenter.php
index ce8cc36..2d884ef 100755
--- a/Interfaces/GSD/Model/GSDCostCenter.php
+++ b/Interfaces/GSD/Model/GSDCostCenter.php
@@ -141,7 +141,7 @@ class GSDCostCenter implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Interfaces/GSD/Model/GSDCostObject.php b/Interfaces/GSD/Model/GSDCostObject.php
index 04f8cc0..8c61f34 100755
--- a/Interfaces/GSD/Model/GSDCostObject.php
+++ b/Interfaces/GSD/Model/GSDCostObject.php
@@ -141,7 +141,7 @@ class GSDCostObject implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/ExchangeLog.php b/Models/ExchangeLog.php
index c01e6cc..95df9c9 100755
--- a/Models/ExchangeLog.php
+++ b/Models/ExchangeLog.php
@@ -184,7 +184,7 @@ class ExchangeLog implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }