Thanks for your response. I had already tried this but was hoping to avoid using GetSelectedCount since, as the user makes his selections, it's possible that he will not make a selection from all fields, in which case, the number selected will be 0, but the count will be more than 1. However, I see that GetSelectedCount does not lead to an invalid dimension message, so this might be the only way, along with a message telling the user to select at least one from each filter.
Here is the result using if(GetSelectedCount(...
Here is the result using if(Count(....
Do you have any idea why using the same formula but just switching GetSelectedCount() for Count() would cause this behaviour. Without further information, it looks like a bug to me.