set number of user slots back to unlimited for a voice channel

I've set the maximum number of users on some of my channels to 5. But when I wanted to switch them back to unlimited, I've got sometimes an error message telling me that the limitation must be greater than or equal to 0. 

However, the error does not happen every time, most of the time it works correctly, but I would like to point out the malfunction.

attached screenshot (in french) of the error.