I just did exactly what you're showing and it allowed me to demount it by clicking on the "Demount"
I know it might be a bit confusing but I believe this is because of the fact that we internally zero'd the cost and instead of reworking the button just set the value to "0" let me know if you're still having the problem.
Essentially, it is asking you to "demount" or "sell" and you have to select the option first.
-Hapa