diff --git a/b_asic/signal_flow_graph.py b/b_asic/signal_flow_graph.py
index 54cf0d18044057233344a0882ae17dea62055298..8e87c71ab59c7eb2cfdce771e169676fbd72f6ce 100644
--- a/b_asic/signal_flow_graph.py
+++ b/b_asic/signal_flow_graph.py
@@ -19,6 +19,7 @@ from typing import (
     Optional,
     Sequence,
     Tuple,
+    Union,
 )
 
 from graphviz import Digraph