diff --git a/Models/Bill.php b/Models/Bill.php
index bdc6a76..7fccbec 100755
--- a/Models/Bill.php
+++ b/Models/Bill.php
@@ -763,7 +763,7 @@ class Bill implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/BillElement.php b/Models/BillElement.php
index a5bf65e..2a3278d 100755
--- a/Models/BillElement.php
+++ b/Models/BillElement.php
@@ -187,7 +187,7 @@ class BillElement implements \JsonSerializable
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }
diff --git a/Models/BillTypeL11n.php b/Models/BillTypeL11n.php
index a3f814d..a1c8f8e 100755
--- a/Models/BillTypeL11n.php
+++ b/Models/BillTypeL11n.php
@@ -129,7 +129,7 @@ class BillTypeL11n implements \JsonSerializable, ArrayableInterface
     /**
      * {@inheritdoc}
      */
-    public function jsonSerialize()
+    public function jsonSerialize() : mixed
     {
         return $this->toArray();
     }