diff --git a/Fill.py b/Fill.py
index abdad440..d4df9fdd 100644
--- a/Fill.py
+++ b/Fill.py
@@ -450,6 +450,12 @@ def distribute_early_items(multiworld: MultiWorld,
 
 def distribute_items_restrictive(multiworld: MultiWorld,
                                  panic_method: typing.Literal["swap", "raise", "start_inventory"] = "swap") -> None:
+    assert all(item.location is None for item in multiworld.itempool), (
+        "At the start of distribute_items_restrictive, "
+        "there are items in the multiworld itempool that are already placed on locations:\n"
+        f"{[(item.location, item) for item in multiworld.itempool if item.location is not None]}"
+    )
+
     fill_locations = sorted(multiworld.get_unfilled_locations())
     multiworld.random.shuffle(fill_locations)
     # get items to distribute