diff --git a/telethon/events/chataction.py b/telethon/events/chataction.py
index 132f2782..3452889e 100644
--- a/telethon/events/chataction.py
+++ b/telethon/events/chataction.py
@@ -36,8 +36,10 @@ class ChatAction(EventBuilder):
                                          added_by=True,
                                          users=msg.from_id)
             elif isinstance(action, types.MessageActionChatAddUser):
+                # If an user adds itself, it means they joined
+                added_by = ([msg.from_id] == action.users) or msg.from_id
                 event = ChatAction.Event(msg,
-                                         added_by=msg.from_id or True,
+                                         added_by=added_by,
                                          users=action.users)
             elif isinstance(action, types.MessageActionChatDeleteUser):
                 event = ChatAction.Event(msg,