diff --git a/app/code/core/Mage/Catalog/Model/Category/Api.php b/app/code/core/Mage/Catalog/Model/Category/Api.php
index f7b7cb1679ba5..b48a364604530 100644
--- a/app/code/core/Mage/Catalog/Model/Category/Api.php
+++ b/app/code/core/Mage/Catalog/Model/Category/Api.php
@@ -446,7 +446,7 @@ public function assignProduct($categoryId, $productId, $position = null, $identi
     {
         $category = $this->_initCategory($categoryId);
         $positions = $category->getProductsPosition();
-        $productId = $this->_getProductId($productId);
+        $productId = $this->_getProductId($productId, $identifierType);
         $positions[$productId] = $position;
         $category->setPostedProducts($positions);