Frame property definition defined in code does not show correct editor

Description

Steps to reproduce:

1. Add a Frame property for a content type in code:

        [BackingType(typeof(PropertyFrame))]
        [UIHint("TargetFrame")]
        public virtual int Frame { get; set; }

2. Rebuild and start the test site.

Expected: The site lets you edit the Frame property with no default value.
Actual: Shows the regular number editor.