Parameters in SingleValueQuery function

Hi,

I've seen this question posted before but the solution given didn't work for me for some reason?

I have a parameter in my report ${monthdate} (String) which is working correctly in the main query for the report. I need to use this same parameter to display the Month name in the report header.

I am using a Single Value Query function, MonthYearFunction, and have created a query in the JDBC data source as such:

SELECT concat(MONTHNAME(${monthdate}),' ', Year(${monthdate}))

The I reference the function in the report header : Data for $(MonthYearFunction) but nothing is displayed.

It works perfectly if I replace the parameter with a string ('2013-06-01') but the parameter being passed to the Single Value Query function seems to be null?

Sorry I am very new to Pentaho and am possibly doing something quite stupid :) There is probably a much better way to do this, and any help I could get would be really appreciated!

Cheers,

Ben.