sync engine update workflow

An Action workflow is triggered when engine updates last name.   The action workflow checks for a condition which is always returned false. Not sure what I am doing wrong. Please help.  

MPR - sync engine updates lastname, call the action workflow.

workflow -If old lastname is equal to professionalname, update new lastname to professional name. If its not equal, dont do anything.

IIF(Eq([//Target/LastName],[//Target/ProfLastName]),[//Delta/LastName], Null())

Allow Null is not checked.

Thanks in advnce.