Disallowed key characters in global data.

[llgdd]

I have the following code. Everything runs pretty smooth, except for one thing: shots, produced by the lib are transparent and contain nothing but Disallowed key characters in global data. at the top. Does anybody have an idea what it means and what causes it?

var options = {
   screenSize: { width: 960, height: 1024 },
   shotSize:   { width: 960, height: 'all'}
}
webshot(url, filePath, options, function(err) {
   if (err) throw err
   console.log("Webshot success!")
   sendShot()
})