diff --git a/test/general/test_implemented.py b/test/general/test_implemented.py
index 67d0e5ff..b60bcee4 100644
--- a/test/general/test_implemented.py
+++ b/test/general/test_implemented.py
@@ -1,6 +1,8 @@
 import unittest
-from worlds.AutoWorld import AutoWorldRegister
 
+from Fill import distribute_items_restrictive
+from NetUtils import encode
+from worlds.AutoWorld import AutoWorldRegister, call_all
 from . import setup_solo_multiworld
 
 
@@ -31,3 +33,17 @@ class TestImplemented(unittest.TestCase):
                     for method in ("assert_generate",):
                         self.assertFalse(hasattr(world_type, method),
                                          f"{method} must be implemented as a @classmethod named stage_{method}.")
+
+    def test_slot_data(self):
+        """Tests that if a world creates slot data, it's json serializable."""
+        for game_name, world_type in AutoWorldRegister.world_types.items():
+            # has an await for generate_output which isn't being called
+            if game_name in {"Ocarina of Time", "Zillion"}:
+                continue
+            with self.subTest(game_name):
+                multiworld = setup_solo_multiworld(world_type)
+                distribute_items_restrictive(multiworld)
+                call_all(multiworld, "post_fill")
+                for key, data in multiworld.worlds[1].fill_slot_data().items():
+                    self.assertIsInstance(key, str, "keys in slot data must be a string")
+                    self.assertIsInstance(encode(data), str, f"object {type(data).__name__} not serializable.")