1 files changed, 2 insertions, 2 deletions
diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 18cd2b62f0..0e98db22b3 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -1504,8 +1504,8 @@ class AuthHandler(BaseHandler):
     @staticmethod
     def add_query_param_to_url(url: str, param_name: str, param: Any):
         url_parts = list(urllib.parse.urlparse(url))
-        query = dict(urllib.parse.parse_qsl(url_parts[4]))
-        query.update({param_name: param})
+        query = urllib.parse.parse_qsl(url_parts[4], keep_blank_values=True)
+        query.append((param_name, param))
         url_parts[4] = urllib.parse.urlencode(query)
         return urllib.parse.urlunparse(url_parts)