Getting error on set-mailboxfolderpermissions.

We have some shared resources like walk stations that we want everyone to be able to see the calendar on. 
The current permissions are:

PS h:\psscripts> Get-MailboxFolderPermission -Identity walkstation2:\calendar

FolderName           User                 AccessRights                                                                                                                             
----------           ----                 ------------                                                                                                                                   
Calendar             Default              {Reviewer}                                                                                                                            Calendar             NT User:S-1-5-21-... {Reviewer}                                                                                                                    Calendar             NT User:S-1-5-21-... {Owner}          

I believe this should work but many people are reporting when they try and view this calendar they get an red exlamation point icon with a calendar could not be updated error message. 

So as a test I tried to change the default permissions to Owner. I found I get the error below if i try to change the default permissions to anything other than reviewer. 

PS h:\psscripts> set-MailboxFolderPermission -identity walkstation2:\calendar -User default -AccessRights owner
WARNING: An unexpected error has occurred and a Watson dump is being generated: Object reference not set to an instance of an object.
Object reference not set to an instance of an object.
    + CategoryInfo          : NotSpecified: (:) [Set-MailboxFolderPermission], NullReferenceException
    + FullyQualifiedErrorId : System.NullReferenceException,Microsoft.Exchange.Management.StoreTasks.SetMailboxFolderPermission
    + PSComputerName        : elk.pdinet.com