Hello, I am having trouble with Process Builder being able to recognize null fileds. What I'm trying to do is have an email alert sent 14 days after the Invoice is received, if a PO number has not yet been assigned (no value in the PO Number field) - see pic below

I have set the following conditions:

• PO Number ISNULL Boolean True
• Invoice Received equals yes

However, when testing this process in Dev, it still sends email alerts even when I have assigned a PO Number to the issue. I only want it to do this is there is no PO Number. I have also tried a couple other conditions to recognize this, none of which I could get to work:

• Is null > Boolean > false
• Equals > GlobalConstant >$GlobalConstant.Null
• Custom formula > PO Number__c = null

Additionally, when I do get an email alert, it sends two identical emails.

Any help would be greatly appreciated!