ApiTestCase类请求函数返回“函数名必须是字符串”错误

Question

I'm currently doing integration testing for my small personal project on Slim - testing with PHPunit. I set up my own test class extending the ApiTestCase class.

I only tried it with simple code and PHPStorm is not indicating any errors. I'm on PHP 7.

class SQLTest extends ApiTestCase
{
    public function test_if_successful(){
        $this->request('POST', '/v1/users/login', ['email' => '[e-mail]', 'password' => '123']);
        $this->assertThatResponseHasStatus(200);
    }
}

When run on PHPunit, it gave me an error saying the function name must be a string on the line where I call the request() function.

Here is the request() function in the ApiTestCase class.

protected function request($method, $url, array $requestParameters = [])
    {
        $request = $this->prepareRequest($method, $url, $requestParameters);
        $response = new Response();

        $app = $this->app;
        $this->response = $app($request, $response);
    }

The error pointed is at the last line:

$this->response = $app($request, $response);