Problems with JS implementation

I just thought I'd add another item I've just come across, rather than starting a new post (unless you have a preferred way of people reporting JS syntax issues?). In JS, it's common practice to namespace your code, which can be done in a number of ways, but the simplest way by far is

var com = com || {};
com.mynamespace = com.mynamespace || {};
com.mynamespace.MyClass = function(){
    ...
};

This currently doesn't work however as rather than just reporting the variable as null and then falling back to the blank object, it actually errors with "Field or method does not already exist".

Matt