diff --git a/discord/message.py b/discord/message.py
index e516fe505..05b0da183 100644
--- a/discord/message.py
+++ b/discord/message.py
@@ -610,6 +610,11 @@ class MessageReference:
         .. versionadded:: 2.5
     message_id: Optional[:class:`int`]
         The id of the message referenced.
+        This can be ``None`` when this message reference was retrieved from
+        a system message of one of the following types:
+
+        - :attr:`MessageType.channel_follow_add`
+        - :attr:`MessageType.thread_created`
     channel_id: :class:`int`
         The channel id of the message referenced.
     guild_id: Optional[:class:`int`]