Hi Alex. I tried your suggestion and got the same error message in the query. I simplified it even more, using

IIf([RoomID]=2,[BasicTourCost],"NOT CALCULATED")

I got the same error message.

I then changed the join type between the tables in the query to show ALL records from the TourT, irrespective of whether there was a booking.

When I re-ran the query I saw the text message, from the ELSE part of the IIF function was being displayed. The problem was only occurring when a booking was made.

To cut a very long story short, when I examined the field properties of the RoomID in the TourXClientT (where it is a foreign key) I found it set to TEXT instead of NUMBER. This prevented the relationship link being set up properly and was the cause of the whole problem.

Such a simple thing caused me to waste many hours but it's a lesson I will never forget. Thank you so much for your suggestion that finally helped me to identify the cause of the error message.

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Reply from Alex Hedley:

Glad you got it working, Number/Text issues can occur lots so it's a good one to keep an eye out for.