diff --git a/python/dune/codegen/blockstructured/accumulation.py b/python/dune/codegen/blockstructured/accumulation.py
index 8f10521ff01777b520a46a256e02f55332be0314..9af9759d5e3426df12a10db39f94048e48006867 100644
--- a/python/dune/codegen/blockstructured/accumulation.py
+++ b/python/dune/codegen/blockstructured/accumulation.py
@@ -125,7 +125,7 @@ def generate_accumulation_instruction_vectorized(expr, visitor):
     accumvar_alias = name_accumulation_alias(accumvar, test_lfs)
 
     predicates = boundary_predicates(visitor.measure, visitor.subdomain_id)
-    predicates.append(blockstructured_boundary_predicated(visitor.measure, visitor.subdomain_id))
+    predicates = predicates.union(blockstructured_boundary_predicated(visitor.measure, visitor.subdomain_id))
 
     quad_inames = visitor.quadrature_inames()
     lfs_inames = visitor.test_info.inames