Team Explorer's "Compare With Unmodified Version" is disabled

I'm using VS2010 professional with TFS; in the Team Explorer window, when I right-click and say check-in changes, I see a list of my changes; for some reason, when I right-click and choose Compare > With Unmodified Version is disabled (greyed out).

This is NOT for a new file; the Change type is Edit, so it shou开发者_C百科ld theoretically work.

What am I missing? Why can't I do the comparison?

This feature is used when you are comparing files inside of a shelveset.

If you go to File -> Shelveset -> Unshelve Pending Changes and select one of the shelvesets and then push the Details button you will see in the compare menu that the option "Compare With Unmodified Version" exists on files marked as "edit"