I'm trying to create an XML-file from the values in the input fields, but gets error message

I'm trying to create an XML-file from the values in the input fields, but gets error message

I'm trying to create an XML-file of the values from the input fields:

But I'm getting the error message:
"Failed to execute 'serializeToString' on 'XMLSerializer': parameter 1 is not of type 'Node'.".

The error message doesn't show very well in JSFiddle because of the text almost has the same color as the background.

So what could be wrong? Is there more to it than what is already in the code?
Am I missing something?