Error during websocket handshake 'Connection' header value is not 'upgrade'

[spj-uk]

We are using peerjs with reverse proxy on IIS.

All works lovely on browsers except Safari on iOS. We get the following error:

Error during websocket handshake 'Connection' header value is not 'upgrade'

const peer = new Peer(peerID, { host: 'xxx.yyy.co.uk', path: 'node/urlrewrite/peer/server' });

Does not hit open event, it just goes straight to the close event? Has anyone else experienced this?