Hi Jack,

Based on what I know it happens when the cells are pre-formatted as
text before you entered your formula there. It typically happens when
you download data from some system and try to do formulae in there.
What I do is this - before I enter my formula I change the cell format
to General and then enter my formula and then it works. When I copy
and paste this down it still works for me.

Try it. Hope this helps.

Regards,
- AG