Microsoft Security Essentials: MpCmdRun.exe -SignatureUpdate -unc switch does not work

Microsoft Security Essentials: MpCmdRun.exe -SignatureUpdate -unc switch
does not work

Looking here I can see extracting a downloaded mpam-fe.exe and copying the
files to a shared folder should allow me to run the following command from
an MSE client so it will pull down the updates from the share:
MpCmdRun.exe -SignatureUpdate -"\myserver\MSE-Defs"
This does not work. When I run this and check the log
(C:\Users\ME\AppData\Local\Temp\MpCmdRun.log) I see:
MpCmdRun: Command Line: "C:\Program Files\Microsoft Security
Client\MpCmdRun.exe" -SignatureUpdate -"\\myserver\mse\"
Start Time: ýFri ýSep ý27 ý2013 08:52:07
Start: MpSignatureUpdate()
Update started
Search Started (MU/WU update) (Path: http://www.microsoft.com)...
So its totally ignoring the switch and going out to
http://www.microsoft.com. If I remove the default gateway from this
computer's NIC config the update fails.
How do I actually make MSE pull down definitions from a UNC path?
edit: I know I can just run mpam-fe.exe and it will update, but I'm still
curious about how to make the unc switch work