Changing value of Param

I have a report that passes a date (time stamp) to a subreport. Sometimes the date being passed is  null (or an empty string) and the report . When this happens get an "Incompatible java.lang.String value assigned to parameter GradDate1 in the ColdFusionReport dataset.". I want to change the value of the Param being passed to current date.  Where and How can I change the date in the parama being passed. I've tried many things (functions, in the query, etc) and can't get anything to work. We are on CF 9. Please help!