Display logic following slider question: can't select 'don't know' as trigger

I have a number of slider questions and have included a 'don't know' option for each one. When a respondent picks 'don't know', I would like to ask a follow-up question. However, display logic doesn't seem to allow me to do this: it only offers me options that: 1. have to do with the slider response (less than, equal to, etc.) 2. don't seem to work for the Don't Know button (I've tried 'Contains', but it doesn't work if I type in 'don't know') 3. seem irrelevant ('Matches RegEx' 'Is Displayed' 'Is not displayed') I would be very grateful if someone could let me know whether and how this is possible.