Why PHPUnit does not assert correctly a request with an unvalid method?

what I am trying to do is asserting a request with an unvalid method.

What I testing is a Guest attempting to logging out as GET (unvalid).

This is the code:

/**
 * Enables router filters.
 *
 * @return void
 */ 
public function enableRouterFilters()
{
    $this->app['router']->enableFilters();
}

[...]

/**
 * Tests users/logout route as a Guest with GET.
 *
 * @return void
 */
public function testRouteToUsersLogoutAsGuestWithGetMethod()
{
    $this->enableRouterFilters();

    $response = $this->call('GET', '/users/logout');
    $this->assertResponseStatus(405);
}

[...]

Instead of asserting true, PHPUnit complains about it:

[...]

There was 1 error:

1) RoutesTest::testRouteToUsersLogoutAsGuestWithGetMethod
Symfony\Component\HttpKernel\Exception\MethodNotAllowedHttpException:

[...]

FAILURES!
Tests: 7, Assertions: 11, Errors: 1.

Why it gives me an error if the assertion should be indeed true?