--- a/dom/media/MediaManager.cpp
+++ b/dom/media/MediaManager.cpp
@@ -219,17 +219,17 @@ HostHasPermission(nsIURI &docURI)
   */
   do {
     end = domainWhiteList.FindChar(',', begin);
     if (end == (uint32_t)-1) {
       // Last or only domain name in the comma separated list
       end = domainWhiteList.Length();
     }
 
-    rv = idnService->ConvertUTF8toACE(Substring(domainWhiteList, begin, end),
+    rv = idnService->ConvertUTF8toACE(Substring(domainWhiteList, begin, end - begin),
                                       domainName);
     if (NS_SUCCEEDED(rv)) {
       if (HostInDomain(hostName, domainName)) {
         return true;
       }
     } else {
       NS_WARNING("Failed to convert UTF-8 host to ASCII");
     }