Problem with Method parameter that becomes Disk freespace

[Alan Moffet]

CFEngine 2.1.9

It seems obvious, but it doesn't work the way I anticipated.  Passing a 
parameter into the method that becomes a freespace variable of a disks 
action does not work.  The action complains about the variable:

cfengine::doCheckDiskSpace:
cf:cfengine::/var/cfengine/modules/cf.diskspace:28: disk/required size 
attribute with silly value (must be > 0)

Quoting the argument doesn't help.

Can anyone suggest what I have done wrong?

I'm invoking the method like this:

methods:

      doCheckDiskSpace(6000mb)

         action=cf.diskspace
         returnvars=result

     returnclasses=fail
         server=localhost

The method is defined this way:

control:
     any::

       MethodName          = ( doCheckDiskSpace )
       MethodParameters    = ( fs )

       actionsequence      = ( disks )

disks:
   any::
       /tmp   freespace=$(fs) ifelapsed=0 inform=true force=true
define=fail_nfd