diff --git a/anki/deck.py b/anki/deck.py
index a75a8a1ac..e2c0d3788 100644
--- a/anki/deck.py
+++ b/anki/deck.py
@@ -357,6 +357,7 @@ where isDue = 1""")
             elif ease == 4:
                 factor = card.factor * self.factorFour
                 interval = (card.interval + delay) * factor
+            assert card.fuzz
             interval *= card.fuzz
         if self.maxScheduleTime:
             interval = min(interval, self.maxScheduleTime)