diff --git a/src/Message/AbstractRequest.php b/src/Message/AbstractRequest.php
index 003bd76..bf02e9f 100644
--- a/src/Message/AbstractRequest.php
+++ b/src/Message/AbstractRequest.php
@@ -220,7 +220,7 @@ protected function getBaseData()
             $data['customer_vault'] = $this->customer_vault;
         }
         
-        if (isset($this->getSecurityKey())) {
+        if ($this->getSecurityKey() !== null) {
             $data['security_key'] = $this->getSecurityKey();
         } else {
             $data['username'] = $this->getUsername();