Unable to deactivate Amumu's W when oom

I was recently in a ranked game where I was unable to deactivate Amumu's Despair while out of mana, with the icon being highlighted light blue but still constantly draining mana and leaving me at 0. I was only able to deactivate it by returning to fountain and exceeding 8 mana for the toggle. This happened several times in the same game but I have been unable to reproduce it by myself in custom games. If anything, I highly suspect it has something to do with being silenced while running out of mana. Has anyone else run into the same problem before?