That validation is for text fields and multiselects. Not sure why it's applying to your select field. Was it a text field at some point? If so you may have leftover parameters from the text field causing that to trigger. Change your field to a query multiselect then under Global set "Maximum Select" to 0 then Parameters > Validation set "Minimum Select" to 0, Save, then change back to a query single select. That should fix those parameters and have added a bug ticket to add checks for this internally so it won't cause problems like this.
forge.joomlapolis.com/issues/6447