Thanks Jason,

I'm no longer convinced that my suggestion was good. As Nando said, I was
perhaps trying to treat NULL as a value. I don't want to start another long
thread, only to defend myself a bit: I was trying to get results of a left
outer join for those detail records where no key in master exists (0:N
cardinality), and having already a similar TIB_Query set up with a parameter
for the value of the key, I thought I would reuse it for this case, too.
I'm not qualified to say whether it's a good idea to implement automatic
translation of cleared TParam values into IS NULL clauses.
I leave it up to you and the community.

Cheers
TOndrej