We're calling discover the following way where this._wemo is the wemo client instance without any specific options:

this._wemo.discover((error, deviceInfo) => {
   ...
});

From time to time, not always, according to our logs, this has the error parameter set with the error:

Non-whitespace before first tag

This seems to come from xml2js:
Leonidas-from-XIV/node-xml2js#345
Leonidas-from-XIV/node-xml2js#390

Unfortunately I can't reproduce this reliably so I can't really debug it.