Value cannot be null For Parameter name: serverSettings

Here i create a new session and run the poweshell cmdlets in that session,

Cmdlets,

         $s=New-PSSession -ComputerName "Name" [Here the session gets created]

         invoke-command -Session '$s- scriptblock {Get-Mailbox adminuser} [Here the error occurs]

Error:

Value cannot be null.      
Parameter name: serverSettings
    + CategoryInfo          : NotSpecified: (:) [Get-Mailbox], ArgumentNullException
    + FullyQualifiedErrorId : System.ArgumentNullException,Microsoft.Exchange.Management.RecipientTasks.GetMailbox.

Any body knows y this kind of error occurs????Thanks a lot..........