diff --git a/direct/src/interval/Interval.py b/direct/src/interval/Interval.py
index a82c8701d9..6ea262398d 100644
--- a/direct/src/interval/Interval.py
+++ b/direct/src/interval/Interval.py
@@ -36,6 +36,8 @@ class Interval(DirectObject):
         # if it should be ignored in this case.
         self.openEnded = openEnded
 
+        assert(self.duration >= 0)
+
     def getName(self):
         return self.name