diff --git a/python/dune/perftool/ufl/execution.py b/python/dune/perftool/ufl/execution.py
index ddb5e90c9b4d2bf52c2b576f84204c45515ddec4..9e857d975ba9f3eebb2adcf5a389eca37641a8e9 100644
--- a/python/dune/perftool/ufl/execution.py
+++ b/python/dune/perftool/ufl/execution.py
@@ -140,7 +140,7 @@ def split_expression(expr):
 
 class FiniteElement(ufl.FiniteElement):
 
-    __slots = ufl.FiniteElement.__slots__ + ("dirichlet_expression", "dirichlet_constraints")
+    __slots = ufl.FiniteElement.__slots__ + ["dirichlet_expression", "dirichlet_constraints"]
 
     def __new__(cls, *args, **kwargs):
         kwargs.pop('dirichlet_constraints', None)