Solve RMAN-06056: could not access datafile 91

Got an error said the RMAN backup failed this morning.
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of backup plus archivelog command at 04/02/2009 18:28:00
RMAN-06056: could not access datafile 91
Solution:Add skip INACCESSIBLE
run {
backup AS COMPRESSED BACKUPSET archivelog all delete all input skip INACCESSIBLE;
}