Simple ValidIF problem

[apocunningham]

I have a simple VALIDIF issue that just getting the better of me. Your thoughts and suggestions would be much appreciated. 

I have a column, ConditionsOrdered which is an EnumList type. The default value of this is "None". What I'm finding is that my users are being lazy and not unchecking "None" when a condition is infact ordered. I'd like to set up a ValidIF so that if the user selects any condition in the list other than "None" and fails to uncheck "None" that there is a "invalid entry" error. 

Thanks in advance for all your help!

[Steve · Accepted solution]

OR(
  NOT(IN("None", [_THIS])),
  ISBLANK([_THIS] - LIST("None"))
)

[apocunningham]

THANK YOU SO MUCH!!! So simple and just getting the best of me. Thanks again!