diff --git a/content/base/public/nsIXMLHttpRequest.idl b/content/base/public/nsIXMLHttpRequest.idl
index e43a22a6271a..ea5fb98ec9b8 100644
--- a/content/base/public/nsIXMLHttpRequest.idl
+++ b/content/base/public/nsIXMLHttpRequest.idl
@@ -194,7 +194,7 @@ interface nsIXMLHttpRequest : nsISupports {
    * returns only after the response has been received.
    *
    * @param body Either an instance of nsIDOMDocument, nsIInputStream
-   *             or a string (nsISupportsWString in the native calling
+   *             or a string (nsISupportsString in the native calling
    *             case). This is used to populate the body of the
    *             HTTP request if the HTTP request method is "POST".
    *             If the parameter is a nsIDOMDocument, it is serialized.
diff --git a/content/base/src/nsXMLHttpRequest.cpp b/content/base/src/nsXMLHttpRequest.cpp
index 943f7f9d89b2..8ce585717898 100644
--- a/content/base/src/nsXMLHttpRequest.cpp
+++ b/content/base/src/nsXMLHttpRequest.cpp
@@ -1057,8 +1057,8 @@ nsXMLHttpRequest::Send(nsIVariant *aBody)
           if (NS_FAILED(rv)) 
             return NS_ERROR_FAILURE;
         } else {
-          // nsISupportsWString?
-          nsCOMPtr<nsISupportsWString> wstr(do_QueryInterface(supports));
+          // nsISupportsString?
+          nsCOMPtr<nsISupportsString> wstr(do_QueryInterface(supports));
           if (wstr) {
             wstr->GetData(getter_Copies(serial));
           } else {