I am trying to use a Validation Rule to assure that the user does not enter a value in a field that is already used in another

list in the same site.  In the Validation Rule for the SiteURL field, I have:

not(isNullOrEmpty(lookup("Site Collection Portfolio","Site URL",{ItemProperty:SiteURLFull},"ID")))
Result: NEVER evaluates to true.  I tried values that are and are not in the lookup list.

My thinking was that if the lookup function returned a value for ID, then the validation fails (user is not allowed to save), and

if the lookup function failed to return a value for ID, it would be null or empty, and the validation passes (user is allowed to

save).

A couple of questions:
(1) Should the "List Title" (parameter 1 of the lookup function) be Display Name or Internal Name?  I think it is Display Name

because I receved an error when I tried Internal Name, but I am not sure.
(2) Should the "Column To Filter On" (parameter 2 of the lookup function) be Display Name or Internal Name?
(3) Will this work if the "Value To Filter On" (parameter 3 of the lookup function) is a Calculated Column?  I also tried it as

two concatenated Single Line of Text fields, and got the same results.

Any help is appreciated - thanks!

(Nintex Forms 2.4.1.0)