diff --git a/Models/Invoice.php b/Models/Invoice.php
index 1868c21..5257db4 100644
--- a/Models/Invoice.php
+++ b/Models/Invoice.php
@@ -516,7 +516,7 @@ class Invoice implements \JsonSerializable
         $this->elements[] = $element;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize() : void
     {
 
     }
diff --git a/Models/InvoiceElement.php b/Models/InvoiceElement.php
index 78d33dc..96fa337 100644
--- a/Models/InvoiceElement.php
+++ b/Models/InvoiceElement.php
@@ -273,7 +273,7 @@ class InvoiceElement implements \JsonSerializable
         return $this->totalPriceGross;
     }
 
-    public function jsonSerialize()
+    public function jsonSerialize() : void
     {
 
     }