diff --git a/app/tools.py b/app/tools.py
index 7936153..e7ff34a 100644
--- a/app/tools.py
+++ b/app/tools.py
@@ -204,7 +204,9 @@ def compute_lower_limits_for_new_pitstop(
 
 
 def pitstop_service_key(x):
-    return x.date, x.odometer
+    # if the entry got no odometer (regular costs!) then we assume it's okay
+    # to have regular cost at a virtual odometer of 0 on that day.
+    return x.date, x.odometer or 0
 
 
 def get_event_line_for_vehicle(vehicle):