Is this error message "Cannot back up the log of the master database " critical? how can I resolve this?

This is the alert message that I've recieved. Very often the daily backup will complete with error as follows:

Failure Reason:
"  ERROR CODE [30:325]: Error encountered during backup. Error: [ERROR: [Microsoft][ODBC SQL Server Driver][SQL Server]Cannot back up the log of the master database. Use BACKUP DATABASE instead. [Microsoft][ODBC SQL Server Driver][SQL Server]BACKUP LOG is terminating abnormally.]
Associated Events:
"  79447 [backup00/JobManager] [01/05/2010 22:20:06 ]: Backup job [19777] completed. Client [sql02], Agent Type [SQL Server], Subclient [System DBs], Backup Level [Transaction Log], Objects [2], Failed [1], Duration [03:19:55], Total Size [1.31 MB], Media or Mount Path Used [ITD011L2].
"  79446 [sql02/SQLiDA] [01/05/2010 22:19:51 ]: Error encountered during backup. Error: [ERROR: [Microsoft][ODBC SQL Server Driver][SQL Server]Cannot back up the log of the master database. Use BACKUP DATABASE instead. [Microsoft][ODBC SQL Server Driver][SQL Server]BACKUP LOG is terminating abnormally.]
"  79445 [sql02/SQLiDA] [01/05/2010 22:19:49 ]: Query Result [[Microsoft][ODBC SQL Server Driver][SQL Server]Cannot back up the log of the master database. Use BACKUP DATABASE instead. [Microsoft][ODBC SQL Server Driver][SQL Server]BACKUP LOG is terminating abnormally.].
"  79415 [backup00/JobManager] [01/05/2010 19:00:11 ]: New backup request received for Client [sql02], iDataAgent [SQL Server], Instance [SQL02], Subclient [System DBs], Backup Level [Transaction Log].
Files failed to back up:
"  Backup Database[master] Failed

But sometimes it completed backup with no errors. I would like to know if the above error is critical or how can I fix it? Thank you.