diff --git a/src/wishlist/storage.py b/src/wishlist/storage.py
index ad51f9d..3f67079 100644
--- a/src/wishlist/storage.py
+++ b/src/wishlist/storage.py
@@ -99,7 +99,7 @@ def write_all_reservations(
 
     reservations_json = []
     for r in reservations.values():
-        reservations_json.append({"item_id": r.item_id, "name": r.name})
+        reservations_json.append({"item_id": str(r.item_id), "name": r.name})
 
     with open(path, "w", encoding="UTF-8") as fp:
         json.dump({"reservations": reservations_json}, fp)