SpFarm Backup Error in Powershell script in Sharepoint 2010

Hi ,

I am taking SPFarm backup in sharepoint 2010. But when the command is preparing to  backup of service account it is throwing the following error.

To trobulshoot I am taking the service backup using the below command. But I am getting the error message as below.

Comand

Backup-SPFarm -Directory "c:\spBackup" -BackupMethod Full -Item "Farm\Shared Services\Shared Services Applications\Search Service Application 1" -Verbose

Error

[1/15/2013 4:14:32 PM] Verbose: [Access Services] Stored this object's parent type: Microsoft.Office.Access.Server.MossHost.AccessServerWebService
[1/15/2013 4:14:32 PM] Verbose: [Access Services] Stored application pool name: SharePoint Web Services Default
[1/15/2013 4:14:32 PM] FatalError: Object Access Services failed in event OnPrepareBackup. For more information, see the spbackup.log or sprestore.log file located in the backup directory.
	IdentityNotMappedException: Some or all identity references could not be translated.
[1/15/2013 4:14:32 PM] Debug:    at System.Security.Principal.SecurityIdentifier.Translate(IdentityReferenceCollection sourceSids, Type targetType, Boolean forceSuccess)
   at System.Security.Principal.SecurityIdentifier.Translate(Type targetType)
   at Microsoft.SharePoint.Administration.SPServiceBackupRestoreUtilities.StoreApplicationPoolInformation(SPBackupInformation info, SPIisWebServiceApplicationPool applicationPool)
   at Microsoft.SharePoint.Administration.SPIisWebServiceApplication.Microsoft.SharePoint.Administration.Backup.IBackupRestore.OnPrepareBackup(Object sender, SPBackupInformation info)
   at Microsoft.SharePoint.Administration.Backup.SPBackup.RunPrepareBackup(SPBackupRestoreObject node)

All of my applicationpools are configured with the same service account from which I am taking backup.

Please help me for my problem.

Thanks

Samir