InstallTrigger.compareVersion() always returns -5

Hi,

No matter what I do, InstallTrigger.compareVersion(string componentName, 
string componentVersion) always returns -5 for me.

This worked before, with the same lines of javascript code, but now 
fails in more recent builds (but I don't know when exactly this started 
to fail for me because I have been away for 5 months or so).

Is there anything that I can use, for testing purposes, something that 
should be registered at al times?


My code looks like this:

var ass = Components.classes["@mozilla.org/appshell/appShellService;1"]
.getService(Components.interfaces.nsIAppShellService);
var diffLevel = 
ass.hiddenDOMWindow.InstallTrigger.compareVersion(componentName, 
componentVersion);

A bit of help would be fun and appreciated :-)

TIA,
/HJ