You don't have to go into the code, you just need to edit the recordset behavior after the page is created.
I think the issue is being caused by the spanish language version of DW that you are using.
In the code just near the error message, it is checking for whether an entered value was selected or not, your DW Version must be passing a different string for the selected value which is causing the error.
the work around will be to edit the recordset after the results page is created.