diff --git a/transitfeed/trip.py b/transitfeed/trip.py
index ae9b374c..81b464ae 100755
--- a/transitfeed/trip.py
+++ b/transitfeed/trip.py
@@ -764,4 +764,4 @@ def AddToSchedule(self, schedule, problems):
 
 
 def SortListOfTripByTime(trips):
-  trips.sort(key=GetStartTime)
+  trips.sort(key=Trip.GetStartTime)