diff --git a/masque/repetition.py b/masque/repetition.py
index e1507b8..a8de94c 100644
--- a/masque/repetition.py
+++ b/masque/repetition.py
@@ -397,6 +397,8 @@ class Arbitrary(Repetition):
         Returns:
             `[[x_min, y_min], [x_max, y_max]]` or `None`
         """
+        if self.displacements.size == 0:
+            return None
         xy_min = numpy.min(self.displacements, axis=0)
         xy_max = numpy.max(self.displacements, axis=0)
         return numpy.array((xy_min, xy_max))