TypeError: Object #<ServerResponse> has no method 'en'

I'm running a Node.js service that uses node-soap to convert a soap response from another service to JSON which then gets returned to the caller. It also works in the other directions, turning JSON into Soap and delivering it to the service.

But I'm getting the following error when calling a couple of services:

TypeError: Object #<ServerResponse> has no method 'en'
at C:\...\soap.js:47:11
at C:\...\node_modules\soap\lib\client.js:84:13
at C:\...\node_modules\soap\lib\client.js:150:24
at Request._callback (C:\...\node_modules\soap\lib\http.js:48:13)
at Request.self.callback (C:\...\node_modules\soap\node_modules\request\index.js:148:22)
at Request.EventEmitter.emit (events.js:98:17)
at Request.<anonymous> (C:\...\node_modules\soap\node_modules\request\index.js:876:14)
at Request.EventEmitter.emit (events.js:117:20)
at IncomingMessage.<anonymous> (C:\...\node_modules\soap\node_modules\request\index.js:827:12)
at IncomingMessage.EventEmitter.emit (events.js:117:20)

I've been unable to find anyone posting the same error, nor finding out what this could be.

All help appreciated.