diff --git a/src/Messages/CompleteRequest.php b/src/Messages/CompleteRequest.php
index 8fb7a02..d81eb56 100644
--- a/src/Messages/CompleteRequest.php
+++ b/src/Messages/CompleteRequest.php
@@ -100,7 +100,7 @@ protected function validateResponseParameters()
 
         //check for missing fields, will throw exc. on missing fields
         foreach ($responseFields as $fieldName => $usedInHash) {
-            if (! isset($response[$fieldName])) {
+            if (! array_key_exists($fieldName, $response)) {
                 throw new InvalidRequestException("The $fieldName parameter is required");
             }
         }