Approval rules setup not working for blank value in one parameter

Summary:

I have setup an approval rule for time card approvals and the logic required is as below -

If Job level = 40, then approval to line manager

else if job level <> 40, then approval to HR Specialist role

I have setup this logic but the transaction is failing with the following error - "An attribute in the JobLevel rule has a null value."

The reason is that the employee's job level is blank in system. How to accommodate this scenario? I am not able to use this attribute "job level" because it has blank value for few employees. Technically, above rule "if job level <> 40" condition should pass for this employee.