Rule when one field has a value and one doesn't

I am using Nintex 2013.

I have a Nintex form with 3 people picker fields: Approver1, Approver2, Approver3.  (Approver1 is a required field).

What I want is: if Approver3 has a vaule, then Approver2 must also have a value.

I tried a Validation rule on Approver3 but it doesn't work: not(isNullOrEmpty({Self}) && isNullOrEmpty(Approver2)

What is wrong with my syntax?