diff --git a/app.py b/app.py
index a8c90d9..c3ac5dc 100644
--- a/app.py
+++ b/app.py
@@ -136,6 +136,7 @@ def index():
         habit_lists = []
         name = "Bitte melde dich an."
         heatmap_values = []
+        day = None
 
     # Sort habits by whether they have been checked today and then by slot
     for habit_list in habit_lists: