What's a good strategy for using assert in Promise handlers

I'd like to test the values returned by an async operation using mocha. The values are provided via a plain A+ promise. Unfortunately the .then handler will now swallow all the exceptions thrown by assert so the tests will pass no matter what:

someOp().then(function(result){
  // throws, but will be swallowed
  assert(result.indexOf('I_DONT_WANT_THAT') < 0);
  done();
});

How can I correctly test the result of my async operation? All I can think of is endlessly propagating the error, but at some point I will need assert to throw a real error for my test to fail.