Why "list backup" does not work?

Hi Everyone,
I used rman to connected to catalog database; (with or without set DBID=xxxxx ) and issued "list backup"; but got an error:

RMAN> list backup summary;

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of list command at 09/06/2011 17:00:41
RMAN-06171: not connected to target database

I could not even run "show all" from rman command without getting this same error.

I thought I would not need to connected to target database to obtain my backup record. How come this does not work now?

Thanks.
Yinhai