diff --git a/telethon/telegram_client.py b/telethon/telegram_client.py
index 5d5b3b69..717ec6ac 100644
--- a/telethon/telegram_client.py
+++ b/telethon/telegram_client.py
@@ -2445,7 +2445,7 @@ class TelegramClient(TelegramBareClient):
 
     async def catch_up(self):
         state = self.session.get_update_state(0)
-        if not state:
+        if not state or not state.pts:
             return
 
         self.session.catching_up = True
diff --git a/telethon/utils.py b/telethon/utils.py
index 0ba64177..ddf68cca 100644
--- a/telethon/utils.py
+++ b/telethon/utils.py
@@ -552,8 +552,13 @@ def resolve_id(marked_id):
     if marked_id >= 0:
         return marked_id, PeerUser
 
-    if str(marked_id).startswith('-100'):
-        return int(str(marked_id)[4:]), PeerChannel
+    # There have been report of chat IDs being 10000xyz, which means their
+    # marked version is -10000xyz, which in turn looks like a channel but
+    # it becomes 00xyz (= xyz). Hence, we must assert that there are only
+    # two zeroes.
+    m = re.match(r'-100([^0]\d*)', str(marked_id))
+    if m:
+        return int(m.group(1)), PeerChannel
 
     return -marked_id, PeerChat
 
diff --git a/telethon_generator/generators/tlobject.py b/telethon_generator/generators/tlobject.py
index 63613c9f..e0c1844d 100644
--- a/telethon_generator/generators/tlobject.py
+++ b/telethon_generator/generators/tlobject.py
@@ -477,6 +477,12 @@ def _write_arg_to_bytes(builder, arg, args, name=None):
         # Else it may be a custom type
         builder.write('bytes({})', name)
 
+        # If the type is not boxed (i.e. starts with lowercase) we should
+        # not serialize the constructor ID (so remove its first 4 bytes).
+        boxed = arg.type[arg.type.find('.') + 1].isupper()
+        if not boxed:
+            builder.write('[4:]')
+
     if arg.is_flag:
         builder.write(')')
         if arg.is_vector: