QUdpSocket::bind() failure in case http_proxy environment variable is set (The proxy type is invalid for this operation)

$ http_proxy= ../build-test_qudpsocket_004-Desktop_Qt_5_7_1_GCC_64bit-Debug/test_qudpsocket_004 
Debug: Server::start() - bind(Any, 8123) succeeded
^C
$ http_proxy=http://bla.org ../build-test_qudpsocket_004-Desktop_Qt_5_7_1_GCC_64bit-Debug/test_qudpsocket_004 
Debug: Server::start() - bind(Any, 8123) succeeded
^C
$ http_proxy= ../build-test_qudpsocket_004-Desktop_Qt_5_8_0_GCC_64bit-Debug/test_qudpsocket_004 
Debug: Server::start() - bind(Any, 8123) succeeded
^C
$ http_proxy=http://bla.org ../build-test_qudpsocket_004-Desktop_Qt_5_8_0_GCC_64bit-Debug/test_qudpsocket_004 
Error: Server::errorUdpSocket() - socketError:  QAbstractSocket::UnsupportedSocketOperationError
Error: Server::start() - QUdpSocket::bind(Any, 8123) failed -  The proxy type is invalid for this operation