diff --git a/masque/repetition.py b/masque/repetition.py
index 867adf7..d7b6ac2 100644
--- a/masque/repetition.py
+++ b/masque/repetition.py
@@ -303,7 +303,7 @@ class Grid(Repetition):
         if other.b_vector is None:
             return False
         if not numpy.array_equal(self.b_vector, other.b_vector):
-            return tuple(self.a_vector) < tuple(other.a_vector)
+            return tuple(self.b_vector) < tuple(other.b_vector)
         return False