diff --git a/telethon/_updates/messagebox.py b/telethon/_updates/messagebox.py
index 72081eb9..e5f67f93 100644
--- a/telethon/_updates/messagebox.py
+++ b/telethon/_updates/messagebox.py
@@ -572,8 +572,14 @@ class MessageBox:
         if pts.entry in self.map:
             self.map[pts.entry].pts = pts.pts
         else:
+            # When a chat is migrated to a megagroup, the first update can be a `ReadChannelInbox`
+            # with `pts = 1, pts_count = 0` followed by a `NewChannelMessage` with `pts = 2, pts_count=1`.
+            # Note how the `pts` for the message is 2 and not 1 unlike the case described before!
+            # This is likely because the `pts` cannot be 0 (or it would fail with PERSISTENT_TIMESTAMP_EMPTY),
+            # which forces the first update to be 1. But if we got difference with 1 and the second update
+            # also used 1, we would miss it, so Telegram probably uses 2 to work around that.
             self.map[pts.entry] = State(
-                pts=pts.pts - (0 if pts.pts_count else 1),
+                pts=(pts.pts - (0 if pts.pts_count else 1)) or 1,
                 deadline=next_updates_deadline()
             )