Mocha js tests not showing diff when strings not equal

I have a simple unit test to assert a string is correct but it only shows me the test name that failed, not the diff of expected and actual results. For example, here is some silly test code:

describe('stackoverflow', function(){
    it('should be simple', function() {
        var someValue = 'A' + 1 + 2;
        assert.equal('A3', someValue);
    });
});

The the output isn't very helpful:

$ mocha

  stackoverflow
    1) should be simple   <--- this line is red

Yes I can see which test failed but I can't easily see why. I'd like to see something like:

$ mocha

  stackoverflow
    1) should be simple

       Expected: 'A3'
       Actual  : 'A12'

Is there something I'm doing wrong?