Unit test express-http-proxy with Jest

Question

I have a custom-proxy.ts module that exports a function like this:

import proxy from 'express-http-proxy';

export const customProxy = proxy('a-url', {
  filter: 'POST',
  https: true
}

This is registered with an express route as follows:

server.all('/some/route', customProxy);

I would like to test that proxy is called with the custom arguments using Jest.

I have tried to test it with the following approach:

import proxy from 'express-http-proxy';
import * as CustomProxy from 'custom-proxy';
import request from 'supertest';

describe('test', () => {
  customProxySpy = jest.spyOn(CustomProxy, 'customProxy');

  beforeEach(() => {
    server = express;
    server.all('/some/route', customProxy);
  });

  it('should pass', async () => {
    await request(server).get('/some/route').send();

    expect(customProxySpy).toHaveBeenCalledWith('a-url', {
        filter: 'POST',
        https: true
    });  
  });
});

However the customProxySpy is getting called with IncomingMessage and some headers.

Does anyone know how I can verify the custom arguments get called in proxy()?