Updating a field using workflow - question

Hi,

I'm currently trying to understand the workflow functionality and expression builder in CRM OD. I am trying to get the system to automatically update the "Number of Employees" field on the Account object with "0" (zero) if the field is empty. I have also tried to get it to update with value "1", just to check if there was a particular problem updating it with zero as a value.

I have set up Workflow Rules Condition like this: [<NumberEmployees>] IS NULL
and the Action expression is: [<NumberEmployees>] = 0 in the value field. The Action is type Field Update.
I have also tried [<NumberEmployees>] = 1,  but doesn't help.