diff --git a/ZillionClient.py b/ZillionClient.py
index d518f5ef..1ace8774 100644
--- a/ZillionClient.py
+++ b/ZillionClient.py
@@ -258,6 +258,10 @@ class ZillionContext(CommonContext):
                 assert id_ in id_to_loc
                 self.loc_mem_to_id[mem] = id_
 
+            if len(self.loc_mem_to_id) != 394:
+                logger.warn("invalid Zillion `Connected` packet, "
+                            f"`slot_data` missing locations in `loc_mem_to_id` - len {len(self.loc_mem_to_id)}")
+
             self.got_slot_data.set()
 
             payload = {
diff --git a/worlds/AutoWorld.py b/worlds/AutoWorld.py
index b8329b71..6837d787 100644
--- a/worlds/AutoWorld.py
+++ b/worlds/AutoWorld.py
@@ -251,7 +251,10 @@ class World(metaclass=AutoWorldRegister):
     def fill_slot_data(self) -> Dict[str, Any]:  # json of WebHostLib.models.Slot
         """Fill in the `slot_data` field in the `Connected` network package.
         This is a way the generator can give custom data to the client.
-        The client will receive this as JSON in the `Connected` response."""
+        The client will receive this as JSON in the `Connected` response.
+
+        The generation does not wait for `generate_output` to complete before calling this.
+        `threading.Event` can be used if you need to wait for something from `generate_output`."""
         return {}
 
     def extend_hint_information(self, hint_data: Dict[int, Dict[int, str]]):
diff --git a/worlds/zillion/__init__.py b/worlds/zillion/__init__.py
index 0b9b0e51..069b156c 100644
--- a/worlds/zillion/__init__.py
+++ b/worlds/zillion/__init__.py
@@ -1,6 +1,7 @@
 from collections import deque, Counter
 from contextlib import redirect_stdout
 import functools
+import threading
 from typing import Any, Dict, List, Set, Tuple, Optional, cast
 import os
 import logging
@@ -101,12 +102,15 @@ class ZillionWorld(World):
     """
     my_locations: List[ZillionLocation] = []
     """ This is kind of a cache to avoid iterating through all the multiworld locations in logic. """
+    slot_data_ready: threading.Event
+    """ This event is set in `generate_output` when the data is ready for `fill_slot_data` """
 
     def __init__(self, world: MultiWorld, player: int):
         super().__init__(world, player)
         self.logger = logging.getLogger("Zillion")
         self.lsi = ZillionWorld.LogStreamInterface(self.logger)
         self.zz_system = System()
+        self.slot_data_ready = threading.Event()
 
     def _make_item_maps(self, start_char: Chars) -> None:
         _id_to_name, _id_to_zz_id, id_to_zz_item = make_id_to_others(start_char)
@@ -338,6 +342,7 @@ class ZillionWorld(World):
         zz_patcher.write_locations(self.zz_system.randomizer.regions,
                                    zz_options.start_char,
                                    self.zz_system.randomizer.loc_name_2_pretty)
+        self.slot_data_ready.set()
         zz_patcher.all_fixes_and_options(zz_options)
         zz_patcher.set_external_item_interface(zz_options.start_char, zz_options.max_level)
         zz_patcher.set_multiworld_items(multi_items)
@@ -385,6 +390,7 @@ class ZillionWorld(World):
         assert self.zz_system.randomizer, "didn't get randomizer from generate_early"
 
         rescues: Dict[str, Any] = {}
+        self.slot_data_ready.wait()
         for i in (0, 1):
             if i in zz_patcher.rescue_locations:
                 ri = zz_patcher.rescue_locations[i]