Domain functions and parameter queries

I have a situation where a Dcount function against a complex query
works great on my PC but not on a test PC in an Access 2000 database.
The underlying query is a query based on a query - with a parameter
that is filled via an open form (a date range).

When it errors out on the test PC, it returns an Invalid Use of Null
error - however, there are no null values in the underlying data - no
left joins. Running the query that is used as the basis for the
domain function returns the same data on both PC's.

I found a post from Allen Browne that said dfunctions won't run
against a parameter query - so it sounds like that's my "answer" -
except that it DOES run on my PC!! Hmmm....??? (And Allen is as
close to infallible as people can be, no doubt.)

Given that I have a workaround, one could say this is just a post to
grumble - - BUT, I'm wondering if anyone can shed any further light
on the why of what might be going on!!! Many thanks.

Cindy