>I think the problem is that the expression isn't evaluating. I've tried putting it in the InteractiveChange Event as well.

Perhaps the "Conc"$this.value isn't true?

Maybe you could use: "CONC"$Upper(This.Value)

The 80/20 rule applies to coding the user interface:
80% of your code is to trap 20% (or less) of the possible inputs.

Also: Users will inevitably find the one case you didn't code for.