Sorting contacts by Company; (none) listed twice

Since recently, I’ve switched the view of my Contacts folder to the Phone List view and arranged that view to group the contacts by Company name.

This works great for the contacts for which I’ve defined a company name. However, the contacts for which I haven’t defined a company name are separated in two groups which are both called “Company: (none)”.

Why is this happening and how can I get all these contacts listed in a single “none” group?