RMAN-06004: ORACLE error from recovery catalog database: RMAN-20001: target database not found in recovery catalog

While connecting to the catalog database to run database backup I got this error:

RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of set command at 09/26/2011 14:03:09
RMAN-06004: ORACLE error from recovery catalog database: RMAN-20001: target database not found in recovery catalog

Description:
This error means that this database is not registered in the recovery catalog so you need to register it.

Solution:
$rman target / catalog rman/rman@catalogdb
RMAN> register database;
RMAN> resync catalog;