Workflow to trigger email notification on a blank field

I'm trying to create a workflow that will send an email notification to the opportunity owner when a new opportunity is created and the product purchase price (total opportunity revenue) is blank, which would include opportunities that were converted from a lead. 

The workflow condition below works, but it works on all opportunities created, not just when someone doesn't add a product/purchase price. How do I get it to trigger only on opps where this field is left blank - $0 are an acceptable value.

[{Primary Revenue Amount}] IS NULL

Thank you,

Patti