Problem using %monitor[...]% token in command line...

I have an external process action running that calls a script.  The action is based off of the failure notification for a drive space monitor.  I'd like to get some details from the monitor into my script as arguments (server and drive letter being monitored). 

If I look at the popup XML for my drive space monitor, I can get the server from %monitor[target]%.  I've used this in my failure command line and it evaluates as expected.  But I want to get the drive letter as well.  It looks like %monitor[desc]% will get me the "E$" for the drive letter that is being monitored.  But for some reason that won't evaluate to anything.  Not sure if I'm referencing that token correctly?  Details below.

Monitor Popup XML:

<monitor>

<nv>

  <thold>

   <avgmin>5</avgmin>

   <metric>%</metric>

  </thold>

  <cred>

   <monitoring>336229080434</monitoring>

   <alerting></alerting>

  </cred>

  <ui>

   <name>myserver.mycompany.com E: Drivespace</name>

   <server></server>

  </ui>

  <instance>

   <handle>E$</handle>

   <desc>E$</desc>

  </instance>

  <tactic>rpc</tactic>

  <target>myserver.mycompany.com</target>

</nv>

Failure command line:

-File .\script.vbs -Server %monitor[target]%  -Drive %monitor[desc]%

Evaluates to:

-File .\script.vbs -Server myserver.mycompany.com -Drive

Failure command line:

-File .\script.vbs -Server %monitor[target]%  -Drive "%monitor[desc]%"

Evaluates to:

-File .\script.vbs -Server myserver.mycompany.com -Drive ""

Failure command line:

-File .\script.vbs -Server %monitor[target]%  -Drive %monitor[desc]%STUFF

Evaluates to:

-File .\script.vbs -Server myserver.mycompany.com -Drive STUFF