GUI_ Downlaod Error

Hi Guru's

I am trying to down load fields from different tables from SAP into XLS sheet

but the function module is not calling correctly.... when i execute program i got the following error pls can you help me out

Function Module called incorrectly….

The Call to the function module “ GUI_Download” is Incorrect.

The function Module interface allows you to specify only fields of a particular type under “FILENAME”. The field “W_FILE” specified here is a different field type

Thanks

Yugi