diff --git a/masque/repetition.py b/masque/repetition.py
index 20ec0a3..0426c08 100644
--- a/masque/repetition.py
+++ b/masque/repetition.py
@@ -391,7 +391,9 @@ class Arbitrary(Repetition):
         Returns:
             self
         """
-        self.displacements[:, 1 - axis] *= -1
+        new_displacements = self.displacements.copy()
+        new_displacements[:, 1 - axis] *= -1
+        self.displacements = new_displacements
         return self
 
     def get_bounds(self) -> NDArray[numpy.float64] | None:
@@ -416,6 +418,6 @@ class Arbitrary(Repetition):
         Returns:
             self
         """
-        self.displacements *= c
+        self.displacements = self.displacements * c
         return self