diff --git a/src/Message/CompletePurchaseRequest.php b/src/Message/CompletePurchaseRequest.php
index 89f246f..5689198 100644
--- a/src/Message/CompletePurchaseRequest.php
+++ b/src/Message/CompletePurchaseRequest.php
@@ -12,7 +12,8 @@ class CompletePurchaseRequest extends PurchaseRequest
     protected function generateSignature()
     {
         return sha1(
-            $this->getTransactionReference() . $this->getMerchantId() . $this->getMerchantKey()
+            $this->getTransactionReference() . $this->getShopId() .
+            $this->getMerchantId() . $this->getMerchantKey()
         );
     }