How to spy on nested method in Jasmine?

Consider this usual situation:

var a = {
  b: {
    c: function() {}
  }
}

I want to spy on c and though it's easy with:

spyOn(a.b, 'c');

However it creates a spy but it doesn't work. No errors or so are shown and I can see there's a spy when debugging.

How can I spy on a nested method?

UPDATE

Output is: Object [object Object] has no method 'tohaveBeenCalledWith'