--compare-dest not working for me. what am I doing wrong?

[Lucas Meijer]

Hey.

Can anybody spot my mistake?

mkdir a b c
echo foo > a/test
echo foo > b/test
rsync -av --compare-dest=b a/ c/

Expected results:
no copy of a/test to c/test, since it is already present in b.

Actual results:
a/test gets copied to c/test


What is wrong here? My expectation? my syntax? I'm having the same 
trouble with --link-dest, and --copy-dest. The behaviour I'm seeing
from rsync, is what I would have expected if the --XX-dest= parameter 
would be missing. (I've tried every variation of having or not having 
slashes
after the a/ b/ and c/'s I could think of)

(tested with 2.6.9 on linux, and with cwrsync 2.6.9 on windows xp)

Bye, Lucas