Use an automation to set a custom field value to 'blank

Here's another use case. We have an automation that is triggered by the addition of a designated value to a custom predefined tags field; the automation sets the value of a custom predefined choice list field. I'd like to define the inverse automation rule that when that designated value is removed from the tags field the value of the choice list field is also removed. However, when I create that rule Aha! displays an error.