diff --git a/arelight/backend/d3js/relations_graph_operations.py b/arelight/backend/d3js/relations_graph_operations.py
index f042f9b..6a4eb69 100644
--- a/arelight/backend/d3js/relations_graph_operations.py
+++ b/arelight/backend/d3js/relations_graph_operations.py
@@ -94,9 +94,9 @@ def link_key(link):
 
     nodes = [{"id": id, "c": c} for id, c in used_nodes.items()]
     if operation == OP_DIFFERENCE:
-        basis = list(set(graph_A["basis"])-set(graph_B["basis"]))
+        basis = list(set(graph_A["basis"]).difference(graph_B["basis"]))
     else:
-        basis = list(set(graph_A["basis"])+set(graph_B["basis"]))
+        basis = list(set(graph_A["basis"]).union(graph_B["basis"]))
     equation = "(" + graph_A["equation"] + ")" + OPERATION_MAP[operation] + "(" + graph_B["equation"] + ")"
     result_graph = {"basis": basis, "equation": equation, "nodes": nodes, "links": links}