ActiveSync in Exchange 2010

Hi, I am working my way through Exchange 2010 Training Kit and I have an error. Can anyone help me ?

[PS] C:\Windows\system32>Test-ActiveSyncConnectivity -ClientAccessServer WIN-QQCLPRV1R0O -URLhttp://branyster.co.uk/mai
l -MailboxCredential "nick"

Cannot process argument transformation on parameter 'MailboxCredential'. Cannot convert the "nick" value of type "Syste
m.String" to type "System.Management.Automation.PSCredential".
    + CategoryInfo          : InvalidData: (:) [Test-ActiveSyncConnectivity], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Test-ActiveSyncConnectivity