Undefined behavior if required generator argument is missing

[justindujardin]

In reading the specification, I am not fully clear on what the subscribe method should do in the event that the required generator argument is missing.

1. *generator* is a required argument.
    1. It must be an object.

I would think that specifying the behavior here would be important. As it is now I could choose to build an implementation that returns null or throw an error. It's open to interpretation by the implementer.

What is the correct behavior?