diff --git a/masque/utils/transform.py b/masque/utils/transform.py
index 90647b1..62e0f1e 100644
--- a/masque/utils/transform.py
+++ b/masque/utils/transform.py
@@ -28,8 +28,9 @@ def rotation_matrix_2d(theta: float) -> NDArray[numpy.float64]:
     arr = numpy.array([[numpy.cos(theta), -numpy.sin(theta)],
                        [numpy.sin(theta), +numpy.cos(theta)]])
 
-    # If this was a manhattan rotation, round to remove some inacuraccies in sin & cos
-    if numpy.isclose(theta % (pi / 2), 0):
+    # If this was a manhattan rotation, round to remove some inaccuracies in sin & cos
+    # cos(4*theta) is 1 for any multiple of pi/2.
+    if numpy.isclose(numpy.cos(4 * theta), 1, atol=1e-12):
         arr = numpy.round(arr)
 
     arr.flags.writeable = False