Failing to upgrade from 10.1-RELEASE to 10.3-STABLE

[George Mitchell]

# freebsd-update -r 10.3-STABLE upgrade
Looking up update.FreeBSD.org mirrors... 4 mirrors found.
Fetching metadata signature for 10.1-RELEASE from update5.freebsd.org...
done.
Fetching metadata index... done.
Inspecting system... done.

The following components of FreeBSD seem to be installed:
kernel/generic src/src world/base world/lib32

The following components of FreeBSD do not seem to be installed:
world/doc world/games

Does this look reasonable (y/n)? y

Fetching metadata signature for 10.3-STABLE from update5.freebsd.org...
failed.
Fetching metadata signature for 10.3-STABLE from update4.freebsd.org...
failed.
Fetching metadata signature for 10.3-STABLE from update6.freebsd.org...
failed.
Fetching metadata signature for 10.3-STABLE from update3.freebsd.org...
failed.
No mirrors remaining, giving up.

What am I doing wrong?  (I get the same failure attempting to upgrade
to 10.1-RELEASE and 10.2-RELEASE.)                         -- George