Mocked class not accepting instance of Carbon

I am trying to test a method with PhpUnit and Mockery. In the process of specifying a method should be called with arguments my test fails.

TEST:

 $this->eventRepo = \Mockery::mock('Path\To\EventRepository');

 $start = Carbon::createFromFormat('Ymd-H-i-s', '20141211-09-21-00');
 $end = Carbon::createFromFormat('Ymd-H-i-s', '20141211-09-19-00');

 $this->eventRepo
        ->shouldReceive('findEvent')
        ->withArgs([
            $start,
            $end,
            '1',
            '1234567891'
        ])
        ->andReturn($callEvent);

REAL CODE:

    $start = Carbon::createFromFormat('Ymd-H-i-s', '20141211-09-20-00');
    $end = Carbon::createFromFormat('Ymd-H-i-s', '20141211-09-20-00');

    $event = $this->eventRepo->findEvent(
        $start->subSeconds(60),
        $end->addSeconds(60),
        $id,
        $number
    );

ERROR FROM TEST:

 Mockery\Exception\NoMatchingExpectationException: No matching handler found for EventRepo::findEvent(object(Carbon\Carbon), object(Carbon\Carbon), "1", "1234567891"). Either the method was unexpected or its arguments matched no expected argument list for this method

$this->eventRepo is a mocked in the test. The real code runs correctly. After the error displays, it, I guess var_dump()'s a instance of Carbon.

I have no idea what could be causing this. I tried googling it but not knowing what to google made that pretty worthless. Has anyone run into this before?