QTcpSocket will not connect to localhost without a router

I use QTcpSocket in a client program to connect to a local server application. Both apps are on localhost (127.0.0.1) on the same machine (laptop).   When QTcpSocket::connectToHost() is called to make a localhost connection to the server, it will succeed if the ethernet cable is plugged in to the laptop on which both apps are running. The ethernet cable connects to my router. However, if I unplug the ethernet cable, the connectToHost call reports QAbstractSocket::HostNotFoundError.

I should not need to be connected to the router to access localhost. I am using Qt5.9.2