If you add a 12mm macro extension tube it will work.
I tried a Kenko 12mm tube and a Canon 1.4xIII converter on a 5DIII.
The AF seems to work if the focus point is more than about 18" away.
If I attempt AF within the yellow Macro range indicated on the lens, the AF is jittery and won't lock on. Manual AF works as well as my eyes do
Is this typical for extension tube use?