Enforcing presence of one field based on the contents of another

[BruceTW]

Hi,

What's wrong with this SharePoint field validation?

I get the following unhelpful error message:

[#2]

Well the first thing that looks wrong is no opening parentheses between IF and ISBLANK.

[#3]

Thanks Rob, good spot.

That fixed the formula but then I got a different problem "The formula cannot refer to another column. " Which is a bit annoying and I can't se a way around it.

How do you make a column mandatory only when another column has a particular value?