Twilio logs are showing the following error when Twilio tries to render a TwiML document sent to it by a Python app:

Msg

XML Validation warning

line

10

parserMessage

Invalid content was found starting with element 'Gather'. One of '{Transcription

ErrorCode

12200

Code for the TwiML document is as follows:

<?xml version="1.0" encoding="UTF-8"?>
   <Response>
      <Gather action="/x" numDigits="1" timeout="0">
         {% for file in files %}
            <Play>{{ file }}</Play>
         {% endfor %}
      </Gather>
      {% if start_stream %}
         <Start>
            <Stream url="wss://{{ host }}/y" />
            <Gather numDigits="1" action="/z" timeout="0" />
         </Start>
      {% endif %}
      <Redirect method="POST">/z</Redirect>
   </Response>

It seems the issue is related to the gather verb. I tried different versions of this TwiML document but the error persists. Any help in resolving would be appreciated. Thanks.