Mockery with Laravel don't mocks a method

I use Laravel 5.5 and try to mock a public method of an class, which is used inside this class:

class ToTest {
  public function filters() {
    return 'original';
  }

  public function callMe() {
    return $this->filters();
  }
}

This is my Testcode:

public function it_should_call_bla_bla()
{
  $mock = $this->mock(ToTest::class);
  $mock->shouldReceive('filters')->andReturn('not orignial');

  $toTest = app(ToTest::class);
  print_r($toTest->callMe());
}

private function mock($class)
{
  $mock = Mockery::mock(app($class))->makePartial();

  $this->app->instance($class, $mock);

  return $mock;
}

$toTest->callMe() returns original...