New -None- Field appears out of nothing

New -None- Field appears out of nothing

Wel, basically I've added a custom field to my prospects, it's called SOLD, I've created to possible values for this field:

"Yes" and "No"

After sending data to this account through the CRM API, I've noticed I get a third option which is 
-None-, that third field is messing up my statistics.

Please advice on how I could remove this extra field.

Thank you!