Hi Joni,
Try a search in these forums for "variable passed" as there may be others who have had the same problem in the past. (Are you sure you set the required "list" variable?)
If you still can't get it fixed, we can take a look at your signup form HTML. Please paste the code with the ACTION variable modified to protect your privacy. "example.com" is a good domain to use. Put your code between (code) and (/code) with square instead of round brackets for best results.
Regards