Job Offer approval rule using compensation elements

Summary:

We want to create rules so where a compensation plan is within a job offer it uses one rule and every other offer goes on another rule. Rule for where compensation plan exists works OK, but then the other rule to capture all other offers is failing.

Content (required):

We want to create rules so where a compensation plan is within a job offer it uses one rule and every other offer goes on another rule. Rule for where compensation plan exists works OK, but then the other rule to capture all other offers is failing.

Using something like (Other Compensation Details.Plan Name == null) Any suggestions on rule to say where no compensation plan exists?