Can you post your SQL statement?
The one I'm using for my query looks like this:
SELECT TRefs.tRef, IIf(IsNull([tref]),1,Asc(Left([tref],1))) AS expr1
FROM TRefs
WHERE (((IIf(IsNull([tref]),1,Asc(Left([tref],1))))>64 And (IIf(IsNull([tref]),1,Asc(Left([tref],1))))<91));
It works fime for me...