"""Extract text of a child element. If strict, and child element is
        not present, raises InvalidXMLError and otherwise returns
        None.

        """
        if strict:
            try:
                return self.element.find('sts:{}'.format(name), _STS_NS).text
            except _ETREE_EXCEPTIONS as error:
                raise InvalidXMLError(
                    ('Invalid XML provided for "{}" - erroring tag <{}>. '

                                          preload_content=True,
                                          )
            if response.status != 200:
                message = "Credential refresh failed, response: %s"
                raise CredentialRetrievalError(
                    provider=method,
                    error_msg=message % response.status,
                )

            creds = json.loads(response.data)

            query = {}