pg_upgrade failing from 9.3 to 9.4 because "template0" already exists

Attempting to upgrade a large (>3TB) postgressql database from 9.3 to
9.4 on Ubuntu 14.04 LTS, but the process fails fairly early on. The
error message instructs me to look at the last few lines of
pg_upgrade_utility.log for more info, and the last two lines there (the
only ones that don't succeed) are:

CREATE DATABASE "template0" WITH TEMPLATE = template0 OWNER = "postgres";
psql:pg_upgrade_dump_globals.sql:44: ERROR:  database "template0"
already exists

For reference, the pg_upgrade command as I'm running it is:

/usr/lib/postgresql/9.4/bin/pg_upgrade -k -b /usr/lib/postgresql/9.3/bin
-B /usr/lib/postgresql/9.4/bin -d /DB/postgres/data/9.3/main -D
/DB/postgres/data/9.4/main -o '-c
config_file=/etc/postgresql/9.3/main/postgresql.conf' -O '-c
config_file=/etc/postgresql/9.4/main/postgresql.conf'

Searching google for useful advice yields...little. (Mostly articles
about problems upgrading from 8.4 to 9.2, whose solutions -- when
they're described -- don't help here. Also, there are a number of
sources saying I should just do a pg_dumpall and then a
pg_restore...which would be fine, probably, if I could afford to take my
production database offline for a week.)

I do have a pg_dumpall backup done, and at the moment I'm doing all this
on a test instance, so I can afford to experiment without risking data
corruption or excessive downtime on the production server, but the whole
point of the experimenting is to come up with a procedure that will work
in production when it's time to do it there. Dump-and-restore definitely
does not meet that requirement, even if it does eventually recreate a
working database.

Any ideas? I'm sure I'm missing something obvious, here, but I can't
seem to find any reference that tells me what it would be.