VSTO - Outlook 2007 How to show a user property even if it is missing?

I added a user property to the mail folder by calling it at UserProperty01. Some of the folder items have this property, some do not. I need to show in the folder view if this property is set or not, so I added another UserProperty01Present property and I set it to true when I set the original property and false when I remove it; then i added it to folder view.

The problem is that the little checkbox is checked in those mailboxes where the property is set, empty in those where the property was removed ... and nowhere at all in those where the property was never set in the first place.

My question is, is there a way to show an empty checkbox when the property is not set? The only alternative I have found so far is to scan every mail item and set it to false when not set every time the folder is opened; Of course, I am not very happy with this solution (there may be over 800 emails). Is there another way?

Thank you for your time.