> if Prepared then UnPrepare;
> ParamByName('GradeRef').AsString := xxx;
> ParamByName('GradeName').AsString := S;
> Prepare;
> Open;

Not sure what the UnPrepare is for. Doesn't the first access via ParamByName
prepare the dataset? So the explicit Prepare in line 4 here would be too
late - it probably has no effect and should be removed.

In what line does the error message occur?

Tobias