Optional select list field doesn't allow "None" when issue is edited by Agent

[Patty Yost]

I have a select list field that is optional to allow the user to leave it blank. On the portal screen, the field defaults to "None" and the field value is null when the customer submits the form. All is well until the issue is edited by an agent. On the Edit screen, the field does not display "None" or even a blank -- it displays the first value in the select list, and there is no way to select "None" at this point. If the agent changes anything on the screen and clicks 'Update', the field value is changed to the value displayed on the screen (the first option in the select list).

The only work-around I've found is to remove the field from the Edit screen, but this is not ideal because there may be a legitimate reason for the agent to change this field. Is there another way to accomplish this?