svndiff() is not working..

Hi All,     I have one Master and Mirror svn setup. And this mirror setup will automatically sync in a particular amount of time. And in our client tool we used the svndiff(). When the master and mirror is not in sync we checked out the folder from master at head revision. And then do the relocate option in the checkout folder to mirror database. Now try the svndiff() call with mirror. This is throwing the head revision is not found error bcz the master and mirror is not synced with last commit. After do the svnSync operation svndiff() didn't throw any error. It's working properly.     Is there any way to do the svndiff() without using sync?    Thanks in advance.