When attempting a differential backup of a database, the following error may occur:

This error may occur if:

• The recovery model of the database has been changed to Simple.
• The recovery model of the database has been changed to Full, but no backup has been performed since the change.
• A job using BACKUP DATABASES [*] or BACKUP ALL DATABASES has run when a full backup does not exist for one or more databases on the server.
  This may occur if you have added a database to the server since setting up the job. For more information, see Backing up all databases on an instance.

To resolve this problem, perform a full backup on the database in question.