Test fromEvent rxjs with socket.io ( typescript , jest) - always got undefined object

I am trying to test a FromEvent(rxjs) with socket.io

and in the fromEvent i got always the following data:

["some message","undefined"]

whatever I did it always send the undefined message

its works good when I use the server.on instaed of fromEvent

my FromEvent code:

 onStudent(): Observable<Student> {
    return fromEvent<Student>(this.server, 'students').pipe(map(student => someFunc(student));
  }

someFunc(student:Student){
 here i got the undefined wired message from student var
["some message","undefined"]
}

also happen for sample one:

  onTest(): Observable<any> {
    return fromEvent(this.server, 'test1')
  }

the return is ['message',undefined]

my test code

  import SocketMock from 'socket.io-mock';

  it.only('should emit and recivie', async done => {
      const studentObsService = //...getStudentService
      const socket = new SocketMock();
      socketMock.getSocket = jest.fn(() => Promise.resolve(socket));

      await studentObsService.init();
      let count = 0;
      studentObsService.onStudent().subscribe((value: Student) => {
        
          console.log(value)  // should compare the value with what we sent
        expect(true).toEqual(true);
      });

      await socket.socketClient.emit('students', 'some message');
      await new Promise(resolve => setTimeout(resolve, 1000));
      done();
    });