Participant Custom Field as Multiselect list -- mismatch between documentation and reality

I tried to implement a Participant Custom Field as a Multiselect list, and the documentation says this: Multi-Select List. A drop-down list of pre-defined choices. User may select zero, one, or multiple values.

However, when I implement this, the user is required to select a value.  Is this a bug?  Or is the documentation incorrect?  (I hope it is a bug.  I do not want this to block reviews.)

Mary