diff --git a/src/pymcnp/files/inp/datum/factory.py b/src/pymcnp/files/inp/datum/factory.py
index 16f74a3..660d770 100644
--- a/src/pymcnp/files/inp/datum/factory.py
+++ b/src/pymcnp/files/inp/datum/factory.py
@@ -616,11 +616,10 @@ def create_datum_from_mcnp(source: str, line: types.McnpInteger = None):
 
         case DatumMnemonic.MODEL_PHYSICS_CONTROL:
             tokens.popl()
-
             if tokens:
                 ModelPhysicsControl(tokens.popl())
             else:
-                datum = ModelPhysicsControl('no')
+                datum = ModelPhysicsControl('off')
 
         case DatumMnemonic.LCA:
             tokens.popl()
diff --git a/src/pymcnp/files/inp/datum/model_physics_control.py b/src/pymcnp/files/inp/datum/model_physics_control.py
index 286349e..5fc3319 100644
--- a/src/pymcnp/files/inp/datum/model_physics_control.py
+++ b/src/pymcnp/files/inp/datum/model_physics_control.py
@@ -22,7 +22,7 @@ def __init__(self, setting: str):
             setting: On/Off.
         """
 
-        if setting is None or setting not in {'yes', 'no'}:
+        if setting is None or setting not in {'on', 'off'}:
             raise errors.MCNPSemanticError(errors.MCNPSemanticCodes.INVALID_DATUM_PARAMETERS)
 
         _card.Card.__init__(self, 'mphys')