Multi select field using profile manager

Hello,

I am customizing the register form using profile manager plugin. I need a field to be multi select, luckily, profile manager offers this type of fields, so it does display on the form but whenever i hit the submit button, it says that field must be filled. I did fill it, i have even checked all the options, still displaying the same error.

Does this type of field require any special manip? what to do?

Thank's for your help