apt-get fails with file uri

["Lewis, James M." <Jim.Lewis@alcoa.com>]

I set up a local mirror of potato using rsync.  I put the following
line in sources.list:

deb file:/extra/debian potato main contrib non-free

"apt-get" update works just fine.  However, "apt-get dist-upgrade"
whines about missing files like so:

Reading Package Lists...
Building Dependency Tree...
The following packages will be upgraded
  base-files bin86 pcmcia-cs pcmcia-source urlview 
5 packages upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 0B/994kB of archives. After unpacking 6144B will be used.
Do you want to continue? [Y/n]
Err file: potato/main base-files 2.1.13
  File not found
Err file: potato/main pcmcia-cs 3.1.7-2
  File not found
Err file: potato/main pcmcia-source 3.1.7-2
  File not found
Err file: potato/main urlview 0.7-7
  File not found
Err file: potato/main bin86 0.14.9-3
  File not found
Failed to fetch
file:/extra/debian/dists/unstable/main/binary-i386/base/base-files_2.1.13.de
b
  File not found
Failed to fetch
file:/extra/debian/dists/unstable/main/binary-i386/base/pcmcia-cs_3.1.7-2.de
b
  File not found
Failed to fetch
file:/extra/debian/dists/unstable/main/binary-i386/admin/pcmcia-source_3.1.7
-2.deb
  File not found
Failed to fetch
file:/extra/debian/dists/unstable/main/binary-i386/misc/urlview_0.7-7.deb
  File not found
Failed to fetch
file:/extra/debian/dists/unstable/main/binary-i386/devel/bin86_0.14.9-3.deb
  File not found
E: Unable to fetch some archives, maybe try with --fix-missing?


Note that it's looking for "unstable", not "potato".  I made a link from
potato to unstable and it worked.  Is this a bug or did I miss something
obvious?

tia
jim