supertest - Cannot read property 'address' of undefined

I have the following code in one of my tests:

  request(test.SERVER_URL)
      .post(Endpoint)
      .send( { request: requestPayload })
      .expect(200)
      .end(function(err, res) {
          console.log(res)
          if (err) return done(err);
          done();
      });

and in the endpoint function i'm testing, there's this code:

some logic...
....
const result = db.Query
res.json(result)

It works fine when i run the server and hit the endpoint, but when testing with supertest i get the error:

TypeError: Cannot read property 'address' of undefined

and below it:

'Cannot read property \\'json\\' of undefined

what's wrong?