PowerShell Quota Value

Hi Everyone...

This is a bit of a strange issue on Exchange 2013 

When I ran powershell to get user's quota i get a normal values

PS C:\>Get-Mailbox mgriffin | FL ProhibitSendQuota

ProhibitSendQuota : 1.5 GB (1,610,612,736 bytes)

However, when i want to get the value for each parameter, I get a NULL-VAlue

PS C:\> (Get-Mailbox mgriffin).ProhibitSendQuota.value.toGB()
You cannot call a method on a null-valued expression.

PS C:\> (Get-Mailbox mgriffin).ProhibitSendQuota.value

Returns nothing as if the VALUE is NULL which explains the error above..

Does anyone know why the VALUE of ProhibitSendQuota is NULL even if its set properly?

Thanks