sinon.getCall(0).args[0] is returning the result of the function, not first arg

I am testing a small function using AVA and Sinon. Function looks essentially like this (edited for brevity):

mergeDefaults: function (opts) {
  **console.log('log 1 ->', opts);**
  opts = _.defaultsDeep(opts, defaultOptions);
  return opts;
}

I have written a test to ensure that the correct parameters are passed into _.defaultsDeep.

test.before(t => {
  sandbox = sinon.sandbox.create();
  defaultsDeepSpy = sandbox.spy(_, 'defaultsDeep');
  mergeDefaults(Object.assign({}, testOptions));
});

test('mergeDefaults runs _.defaultsDeep with the correct parameters', t => {
  **console.log('log 2 ->', defaultsDeepSpy.getCall(0).args[0]);**
  t.is(defaultsDeepSpy.getCall(0).args[0], testOptions);
  t.is(defaultsDeepSpy.getCall(0).args[1], defaultOptions);
});

test.after.always(t => {
  sandbox.restore();
});

The problem I am running into is that log 1 and log 2 are not the same. As I understand it, the

spy.getCall(n).args[m]

returns the mth argument passed to the spied on function from the nth call to that function. However, in this case log 2 is actually returning what results from the function, not the first argument.

The options objects look something like this:

testOptions = {
  key1: 11,
  key2: 22,
  key3: 33
}
defaultOptions = {
  key1: 1,
  key2: 2,
  key3: 3,
  key4: 4,
  key5: 5
}

But the console.logs look like this:

log 1 -> testOptions = {
  key1: 11,
  key2: 22,
  key3: 33
}
log 2 -> testOptions = {
  key1: 11,
  key2: 22,
  key3: 33,
  key4: 4,
  key5: 5
}

So the spy.getCall(0).args[0] a.k.a "log 2" is actually returning what the "opts" becomes after the _.defaultsDeep runs. This feels like a bug in the Sinon library, but more likely I am doing something wrong. Help is much appreciated, thanks in advanced!