Could not locate file 'xxx_Log' for database 'master' in sys.database_files

Hi All,
 
I am  truncating Transactionlog…   following the steps mentioned in MSKB.. what is the problem?
 
 
Use masterGo
Backup log xxx with truncate_onlygoDbcc shrinkfile(xxx_Log)Go
Msg 8985, Level 16, State 1, Line 1Could not locate file 'xxx_Log' for database 'master' in sys.database_files. The file either does not exist, or was dropped.
 
Please  help me.
 
Thanks in advance