Unhandled exception on application exit with ofxhttputils

i'm using ofxhttputils to do a periodic http GET. it works fine. but when the application exit, unhandled exception comes out. seems that it is about mutex. does anybody has a solution?
p.s. i also tried the example_basic_client with ofxhttp:

ofx::HTTP::DefaultClient client;
ofx::HTTP::Context context;
ofx::HTTP::BaseResponse response;
ofx::HTTP::GetRequest request(url);
try{
std::istream& responseStream = client.execute(request, response, context);
}

but i guess it is not non-blocking. do we have a non-blocking example for http GET?