diff --git a/api/src/opentrons/protocol_engine/commands/load_module.py b/api/src/opentrons/protocol_engine/commands/load_module.py
index 082b88814cf..1d877d08941 100644
--- a/api/src/opentrons/protocol_engine/commands/load_module.py
+++ b/api/src/opentrons/protocol_engine/commands/load_module.py
@@ -87,7 +87,7 @@ class LoadModuleResult(BaseModel):
     )
 
     serialNumber: Optional[str] = Field(
-        ...,
+        None,
         description="Hardware serial number of the connected module. "
         "Will be `None` if a module is not electrically connected to the robot (like the Magnetic Block).",
     )