diff --git a/src/SIMGateway.php b/src/SIMGateway.php
index 1ef4b4af..973f0316 100644
--- a/src/SIMGateway.php
+++ b/src/SIMGateway.php
@@ -45,7 +45,7 @@ public function completeAuthorize(array $parameters = array())
 
     public function purchase(array $parameters = array())
     {
-        return $this->createRequest('\Omnipay\AuthorizeNet\Message\SIMAuthorizeRequest', $parameters);
+        return $this->createRequest('\Omnipay\AuthorizeNet\Message\SIMPurchaseRequest', $parameters);
     }
 
     public function completePurchase(array $parameters = array())