Expression to Msgbox

[ModerateAccessUser]

This is pretty basic beginner stuff, I am sure. But I cannot seem to get it to work. In the "cell properties" of a cell, I have set the field to use a "look-up" from another table, so the actual input is a number, 1(yes) or 2(No).

I then want access to detect when a user chooses "Yes"or "1" and prompt the user with directions to fill-in another cell. I don't need a prompt for "No" or "2"

Here is what I have tried using in the "validation rule" box on the property sheet:

IIf([Deceased]=1,"You have indicated the patient is deceased. Please enter the patient's decease date.","")

and I keep getting this msg:

Error'Error evaluating CHECK constraint." in the validation rule.

Any ideas?