How to use Report Parameters to limit data

This is what I have tried:

- I added a parameter to the sqlDataSource

- I configured the sqlDataSource to use that parameter.

i.e. Changed the select command to:

SELECT  field1, field2, field3 FROM MyTable
WHERE MyCode = @ReportType

- Rebuilt the solution.

When I click the "Preview" tab I enter my parameter.

I then get an error: ORA-00936 missing expression on the report.

Please advise.