Data Validation and Error Messages

Hi guys/gals

I need to restrict a value in a cell to a choice from a drop down menu, which I'm doing using Data Validation. I also want that to be the only value available to users, so I've set up the error message option to Stop, and the message to tell them to choose from the list.

But I can still put any old value in there without a murmur of an error message! Where am I going wrong?

Help!

TIA

Ah..ignore me. I tried unchecking 'Ignore Blank' and that seems to have worked. If anyone can explain why, that would be a bonus, but it is working!