URI segment disappears after validation_run()

im trying to go from mysite.com/signin/1 to mysite.com/course/enroll/1
but after validation_run() the segments just goes away.

PHP Code:

public function signin(course_id=null) {
  if(!$this->form_validation->run()){
    // wrong input
  }
  else{
    if(!empty($course_id)) {
       redirect(base_url('course/enroll/'.$course_id));
    }
    else {
       redirect(base_url('student'));
    }
  }
} 


i kept on being redirected to mysite.com/student even tho course_id is not empty.

right now, i just used session to get around this problem but i know that is not an efficient solution.