diff --git a/app.py b/app.py
index 22c217c..087d361 100644
--- a/app.py
+++ b/app.py
@@ -632,10 +632,11 @@ def add_user():
 
     # Check if user is already in the habit list
     already = False
-    for u in habit_list.get_users():
-        if u.id == user.id:
-            already = True
-            break
+    if user:
+        for u in habit_list.get_users():
+            if u.id == user.id:
+                already = True
+                break
 
     if already:
         errors['email'] = 'Teilnehmer ist bereits in der Liste.'