diff --git a/Models/Balance.php b/Models/Balance.php
index 67df6b9..5a5ee52 100755
--- a/Models/Balance.php
+++ b/Models/Balance.php
@@ -83,7 +83,7 @@ class Balance
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/BalanceL11n.php b/Models/BalanceL11n.php
index 18349bb..660ae81 100644
--- a/Models/BalanceL11n.php
+++ b/Models/BalanceL11n.php
@@ -122,7 +122,7 @@ class BalanceL11n implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/CostCenter.php b/Models/CostCenter.php
index 8319fb0..132bb87 100755
--- a/Models/CostCenter.php
+++ b/Models/CostCenter.php
@@ -93,7 +93,7 @@ class CostCenter
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/CostCenterL11n.php b/Models/CostCenterL11n.php
index d283729..401c8da 100755
--- a/Models/CostCenterL11n.php
+++ b/Models/CostCenterL11n.php
@@ -122,7 +122,7 @@ class CostCenterL11n implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/CostObject.php b/Models/CostObject.php
index d126795..9d3ab57 100755
--- a/Models/CostObject.php
+++ b/Models/CostObject.php
@@ -93,7 +93,7 @@ class CostObject
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/CostObjectL11n.php b/Models/CostObjectL11n.php
index 420c730..255b575 100755
--- a/Models/CostObjectL11n.php
+++ b/Models/CostObjectL11n.php
@@ -122,7 +122,7 @@ class CostObjectL11n implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/Creditor.php b/Models/Creditor.php
index c7fd6e7..d9e1f0b 100755
--- a/Models/Creditor.php
+++ b/Models/Creditor.php
@@ -68,7 +68,7 @@ class Creditor
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/Debitor.php b/Models/Debitor.php
index b7932c9..d8beae4 100755
--- a/Models/Debitor.php
+++ b/Models/Debitor.php
@@ -68,7 +68,7 @@ class Debitor
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }