diff --git a/masque/ports.py b/masque/ports.py
index 4be208d..93310f7 100644
--- a/masque/ports.py
+++ b/masque/ports.py
@@ -630,7 +630,7 @@ class PortList(metaclass=ABCMeta):
         rotations = numpy.mod(s_rotations - o_rotations - pi, 2 * pi)
         if not has_rot.any():
             if set_rotation is None:
-                PortError('Must provide set_rotation if rotation is indeterminate')
+                raise PortError('Must provide set_rotation if rotation is indeterminate')
             rotations[:] = set_rotation
         else:
             rotations[~has_rot] = rotations[has_rot][0]