diff --git a/api/advent22_api/routers/_security.py b/api/advent22_api/routers/_security.py
index e2c828e..24d4686 100644
--- a/api/advent22_api/routers/_security.py
+++ b/api/advent22_api/routers/_security.py
@@ -57,4 +57,7 @@ async def user_can_view_door(
     True iff das Türchen von Tag `day` user-sichtbar ist
     """
 
+    if day < 0:
+        raise HTTPException(status.HTTP_422_UNPROCESSABLE_ENTITY)
+
     return day < await user_visible_doors()