diff --git a/mozilla/mailnews/imap/src/nsImapIncomingServer.cpp b/mozilla/mailnews/imap/src/nsImapIncomingServer.cpp
index c9be11fe003..7f4a7b767bb 100644
--- a/mozilla/mailnews/imap/src/nsImapIncomingServer.cpp
+++ b/mozilla/mailnews/imap/src/nsImapIncomingServer.cpp
@@ -3762,8 +3762,10 @@ nsImapIncomingServer::GetUriWithNamespacePrefixIfNecessary(PRInt32 namespaceType
       // it may be the case that this is the INBOX uri, in which case
       // we don't want to prepend the namespace. In that case, the uri ends with "INBOX",
       // but the namespace is "INBOX/", so they don't match.
+      nsCAutoString leafName;
+      resultUri.Right(leafName, resultUri.Length() - index - 1);
       if (resultUri.Find(namespacePrefix.get(), PR_FALSE, index+1) != index+1
-        && !Substring(resultUri, index + 1, resultUri.Length() - index - 1).LowerCaseEqualsLiteral("inbox"))
+        && !leafName.EqualsIgnoreCase("inbox"))
         resultUri.Insert(namespacePrefix, index+1);   // insert namespace prefix
       *convertedUri = nsCRT::strdup(resultUri.get());
     }