I have tried reproducing the issue on Vista and MAC 10.6 on CS4 with no luck.
Could you please try to recreate the form on a different page and see if you can reproduce the error.
I would also recommend deleting your configuration and reinstalling the extension:
showpost.php?p=15525&postcount=2
If you are still seeing the error, please post back with the OS and DW version you are running.