diff --git a/b_asic/signal_flow_graph.py b/b_asic/signal_flow_graph.py
index 05e6ad6658e0805734b3b58e2df26eccea40ab11..9d31b04b82509a5749f9899a25cf2091c8492c9e 100644
--- a/b_asic/signal_flow_graph.py
+++ b/b_asic/signal_flow_graph.py
@@ -9,7 +9,7 @@ from b_asic.signal import Signal, SignalSource, SignalDestination
 from b_asic.simulation import SimulationState, OperationState
 from b_asic.graph_id import GraphIDGenerator, GraphID
 
-from typing import List, Dict, Union
+from typing import List, Dict, Union, Optional
 
 class SFG(BasicOperation):
 	"""