Posting XML Data to an API with Requests & ElementTree

I'm currently trying to post an XML string to an API that i've created from a pandas DataFrame using LXML (mainly because ElementTree doesn't have an xml_declaration=True parameter?). However i seem to be receiving an 400 status code within the response.

xml_data = ET.tostring(root, encoding='UTF-8', xml_declaration=True)

url = api_url + version + endpoint

headers = {
  "Content-Type": "application/xml"
}

params = {
  "access_token": access_token
}

response = r.post(url, headers=headers, params=params, data=xml_data)

I have noticed when printing my XML string (and also running it through an XML validator) there is a line break after my XML Declaration that the validator didn't seem to like:

"<?xml version=\'1.0\' encoding=\'UTF-8\'?>\n"

Could it be this causing the 400 error code? If so, how can i remove the line break from the string given it's a "Bytes" data type instead of "String" after the ET.tostring() function?

Or is there anything else noticeably wrong about this xml declaration?

Thanks