node-http2-proxy: Cluster support (Bug??)

I just come across a strange bug here: I wanted to test the speed of the proxys and use the cluster module. Very often I get this error:

connect EADDRINUSE 192.168.1.4:8090
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1141:16) {
  errno: 'EADDRINUSE',
  code: 'EADDRINUSE',
  syscall: 'connect',
  address: '192.168.1.4',
  port: 8090,
  statusCode: 503,
  connectedSocket: false,
  reusedSocket: false
}

But if I only use one worker, I get no error ??? Is http.request compatible with cluster?