Jim Strutz , Aug 20, 1999; 10:57 p.m.
This is fairly common with small aperture zooms. They report f:5.6 because most Canon EOS cameras (and others as well) are designed to NOT allow auto focus with smaller apertures. Canon builds them to reliably AF down to f:5.6 and doesn't want people to complain about erratic performance with slower lenses.
In actual practice most EOS bodies will AF as low as f:8, but only if there is sufficient light and contrast. I wouldn't worry about it. AF performance will not be the greatest but the exposure will still be accurate even with the wrong f:stop being reported.