[AccessD] Report Grouping on a derived query field

Access 2000 - 
In a query which feeds a report I have a derived field.  It is derived directly from a date field in one of the contributing tables, thus:

  FY:Iif(Month(startDate) < 7, Year(startDate), 1+Year(startDate))

The query runs perfectly, and gives me values like 2003, 2004 etc in the FY field.

I am using this derived field, FY, as a grouping level (the only grouping level) in the report.  When I run the report, it bombs off immediately with the error message "The expression is typed incorrectly, or is too complex to be evaluated  ..." and more stuff about simplifying the expression. When I remove the Grouping Level completely the report runs.  There is no code-behind-forms (there will be but I have removed it to attempt to solve this).

Any Ideas?

TIA
Stephen Bond
Otatara, New Zealand