IsNull Formula help

I have two fields, one which is a date and one which is numeric. I would like if either are blank, the action does not fire. Is there a way to set this up?

I am currently trying to use a formula text field and isNull with the following formula:

If(IsNull([Social Insurance Number]) or IsNull([Date of Birth]), "No", "Yes")  

So that I can determine if the action will fire if this formula says yes or no. The issue is if I input a number for [Social Insurance Number] it remains no, though if I input a date for [Date of Birth] if changes to say yes even if [Social Insurance Number] is still blank.