flagstaff_subset is not empty error when using pg_upgrade

sudo -u postgres /usr/lib/postgresql/9.3/bin/pg_upgrade -b /usr/lib/postgresql/9.1/bin -B /usr/lib/postgresql/9.3/bin -d /data/postgres/9.1/main -D /data/postgres/9.3/main -o ' -c config_file=/etc/postgresql/9.1/main/postgresql.conf' -O ' -c config_file=/etc/postgresql/9.3/main/postgresql.conf'
Performing Consistency Checks
-----------------------------
Checking cluster versions                                   ok
Checking database user is a superuser                       ok
Checking for prepared transactions                          ok
Checking for reg* system OID user data types                ok
Checking for contrib/isn with bigint-passing mismatch       ok
Creating dump of global objects                             ok
Creating dump of database schemas
                                                            ok

New cluster database "flagstaff_subset" is not empty
Failure, exiting

I have no idea where to go from here. If anyone knows the root cause of this or has good instructions on upgrading from postgres 9.1 to 9.3, please let me know!