diff --git a/guppy/hugr/ops.py b/guppy/hugr/ops.py
index 970ee163..7d60a6d8 100644
--- a/guppy/hugr/ops.py
+++ b/guppy/hugr/ops.py
@@ -26,7 +26,7 @@ class BaseOp(ABC, BaseModel):
 
     # Parent node index of node the op belongs to, used only at serialization time
     parent: NodeID = 0
-    input_extensions: ExtensionSet = Field(default_factory=list)
+    input_extensions: ExtensionSet | None = None
 
     def insert_port_types(self, in_types: TypeRow, out_types: TypeRow) -> None:
         """Hook to insert type information from the input and output ports into the