It is relatively difficult to call another object's expression, because you need:
1) an instance to call it on - you don't pass any instance, so 'this' in the expression method is not set and ends up being the global scope. Since the method uses 'this' to refer to the object instance, the method fails and causes the javascript error.
2) as
rexrainbow mentioned you need to pass a 'ret' parameter in order to receive the result. This is undocumented and you shouldn't really be doing this in the first place.
Basically, the easiest thing to do is to copy and paste the code from the Mouse plugin.