Comparing "Assigned To" column value to "Current User"

I have a task list, and a Nintex form for that task list. My goal is to disable certain controls on my Nintex form if the "Current User" is not equal to the SharePoint list column "Assigned To".

In one of the form rules, I have the condition set to:

not(equal(Current User, Assigned To))

The result of the rule will be to "disable" the control.

Where "Current User" is from the "Common" tab and "Assigned To" is a SharePoint list column.

When I populate the "Assigned To" column for a particular list item with my domain account and then invoke the edit form for that item, the control is always disabled, regardless of what is contained in "Assigned To".

Any ideas?

Thanks,