diff --git a/ufl/sobolevspace.py b/ufl/sobolevspace.py
index 6dfa83633..d15acda78 100644
--- a/ufl/sobolevspace.py
+++ b/ufl/sobolevspace.py
@@ -111,9 +111,9 @@ def __init__(self, orders):
                 the position denotes in what spatial variable the
                 smoothness requirement is enforced.
         """
-        assert all(
-            isinstance(x, int) or isinf(x) for x in orders
-        ), "Order must be an integer or infinity."
+        assert all(isinstance(x, int) or isinf(x) for x in orders), (
+            "Order must be an integer or infinity."
+        )
         name = "DirectionalH"
         parents = [L2]
         super(DirectionalSobolevSpace, self).__init__(name, parents)