It seems like the "this" seems to be lost in the example.

I'm generating a wrapping function from generator(fn,...) . And then passing the generated function onto a timer to execute later. It crashes a 100% of the time.

The workaround for now is to use "fn.apply({}, args)" in the generator. This issue didn't happen in 5.6.0 if I recall correctly and I have tested this similar code in node.js to double check that this is valid js behavior.