Exchange 2010 SP1 Multi Tenant

When trying to assign full mailbox rights on an Exchange Multi Tenant installation, you receive this error:

Add-MailboxPermission -Identity [email protected] -User userb@domain -AccessRights ‘FullAccess’

The operation on mailbox [email protected] failed because it’s
out of the current user’s write scope. The object ‘userb’ must be within the read scope before and after it’s modified. Can’t perform the save operation.
+ CategoryInfo          : NotSpecified: (ehosting.local/…C/Administrator:ADObjectId) [Add-MailboxPermission], Ta
skInvalidOperationException
+ FullyQualifiedErrorId : CFE6232C,Microsoft.Exchange.Management.RecipientTasks.AddMailboxPermission

The fix to this is to simply log into the hosted exchange as the admin for the organisation and run the command again and it works. :-)