error in transaction backup

I am sometime getting error like this

Destination: [\\yhlbackup\dbbackup\his_db\his_db_tlog_2003062311 00.TRN]
[Microsoft SQL-DMO (ODBC SQLState: 42000)] Error 4213: [Microsoft][ODBC SQL Server Driver][SQL Server]Cannot allow BACKUP LOG because file 'his_db_Data' has been subjected to nonlogged updates and cannot be rolled forward. Perform a full database, or differential database, backup.
[Microsoft][ODBC SQL Server Driver][SQL Server]Backup or restore operation terminating abnormally.

For your more information
I have checked out that nonlogged updates not occurred in database.
And I implemented transactional replication.
This database has two publications and two pull subscriptions also
For further clarification please don’t hesitate …

Anybody can suggest me ..
Thanks in advance for your time.

Vinod

See the database option (recovery model) on the database. something might have changed the option to "Simple mode"