diff --git a/masque/builder/utils.py b/masque/builder/utils.py
index 3109f46..5680694 100644
--- a/masque/builder/utils.py
+++ b/masque/builder/utils.py
@@ -106,7 +106,7 @@ def ell(
             raise BuildError('Asked to find aggregation for ports that face in different directions:\n'
                              + pformat(port_rotations))
     else:
-        if set_rotation is not None:
+        if set_rotation is None:
             raise BuildError('set_rotation must be specified if no ports have rotations!')
         rotations = numpy.full_like(has_rotation, set_rotation, dtype=float)