RMAN restore controlfile error (RMAN-20021: database not set)

RMAN> restore controlfile to '/tmp/controlfile';

Starting restore at 20120321:13:36:17
using channel ORA_DISK_1

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of restore command at 03/21/2012 13:36:17
RMAN-20021: database not set

RMAN> exit

Recovery Manager complete.
-bash-3.2$ NLS_LANG=american_america.we8iso8859p1
-bash-3.2$ rman target /

Recovery Manager: Release 11.2.0.3.0 - Production on Wed Mar 21 13:38:07 2012

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

connected to target database: ORATEST (DBID=3309052188)

RMAN> restore controlfile to '/tmp/controlfile';

Starting restore at 20120321:13:38:11
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=24 device type=DISK

channel ORA_DISK_1: starting datafile backup set restore
channel ORA_DISK_1: restoring control file
output file name=/tmp/controlfile
channel ORA_DISK_1: reading from backup piece +FRA/oratest/autobackup/2012_03_21/s_778511564.295.778511569
channel ORA_DISK_1: piece handle=+FRA/oratest/autobackup/2012_03_21/s_778511564.295.778511569 tag=TAG20120321T131243
channel ORA_DISK_1: restored backup piece 1
channel ORA_DISK_1: restore complete, elapsed time: 00:00:03
Finished restore at 20120321:13:38:17

RMAN>