Exchange 5.5 Backup Error

When I back up Exchange 5.5 Information Store and Directories, using Windows NT 4.0 Backup Utility, I receive the following error in the Event Log:

Microsoft Exchange services returned ‘102’ from a call BackupRead [] ` additional data `-`

Does anyone know what this message means? Judging from the size of the backup it seems that everything is being archived that should be. I am just afraid that the backups will be useless if I ever need to restore from them.