Need help with a Selection formula.

[stormtrooper]

Hello all. I'm working with CR7 and need some help with a selection formula. What I want the report to run off of is this:

if isnull {extension_dt} then {@expiry_dt} else {extension_dt}

Note: {@expiry_dt} = auth_dt + 90

But I get "The result of selection formula must be a boolean."

Any ideas?

Thanks

 

[stormtrooper]

Sorry. This is not what I wanted. Please disregard this post.