the callback passed to evhttp_request_new might get NULL

Question

The following function is a callback passed to evhttp_request_new().

My code sometime abort because of assert fail.
I am confusing that what may cause the pointer req to get NULL?

void http_request_done(struct evhttp_request *req, void *arg)
{
    assert(req); //assert fail here
}

Answer 1

The callback passed to evhttp_request_new might get NULL if there was a timeout.