+    @staticmethod
+    def _check_error(json_string, key_or_keys):
+        keys = [key_or_keys] if isinstance(key_or_keys, compat_str) else key_or_keys
+        for key in keys:
+            error = json_string.get(key)
+            if error:
+                raise ExtractorError('Unable to login: %s' % error, expected=True)
+
+    def _login_step(self, form_html, fallback_action_url, extra_form_data, note, referrer_url):
+        action_url = self._search_regex(
+            r'<form[^>]+action=(["\'])(?P<url>.+?)\1', form_html,
+            'post url', default=fallback_action_url, group='url')
+
+        if not action_url.startswith('http'):
+            action_url = compat_urlparse.urljoin(self._SIGNIN_URL, action_url)
+
+        form_data = self._hidden_inputs(form_html)
+        form_data.update(extra_form_data)
+
+        response = self._download_json(
+            action_url, None, note,
+            data=urlencode_postdata(form_data),
+            headers={
+                'Referer': referrer_url,
+                'X-Requested-With': 'XMLHttpRequest',
+            }, expected_status=(418, 500, ))
+
+        self._check_error(response, ('email', 'password', 'ErrorMessage'))
+
+        return response, action_url
+