WF not working when original entry is blank

I have a WF setup that sends out an email when the value of a particular field is changed. My problem is on the initial change when the value changes from empty to a picklist value the WF does not

fire. After the this field is popluated the first time and is changed the WF works without a problem. Below is my condition, what am I missing to get it to work when changing from empty to a value?

PRE('<plInitiate_Escalation_Process_ITAG>')<>[<plInitiate_Escalation_Process_ITAG>] AND [<plInitiate_Escalation_Process_ITAG>] IS NOT NULL

Any help is appreciated.