diff --git a/src/models/PayflowPaymentForm.php b/src/models/PayflowPaymentForm.php
index 9a4a57d..a984869 100644
--- a/src/models/PayflowPaymentForm.php
+++ b/src/models/PayflowPaymentForm.php
@@ -8,7 +8,7 @@ class PayflowPaymentForm extends CreditCardPaymentForm
 {
     public mixed $cardReference = null;
 
-    public function populateFromPaymentSource(PaymentSource $paymentSource)
+    public function populateFromPaymentSource(PaymentSource $paymentSource) : void
     {
         $this->cardReference = $paymentSource->token;
     }