diff --git a/simpeg/potential_fields/gravity/simulation.py b/simpeg/potential_fields/gravity/simulation.py
index 2c8ac62701..7fcf032cd1 100644
--- a/simpeg/potential_fields/gravity/simulation.py
+++ b/simpeg/potential_fields/gravity/simulation.py
@@ -145,7 +145,7 @@ def __init__(
                 self._sensitivity_gravity = _sensitivity_gravity_serial
                 self._forward_gravity = _forward_gravity_serial
 
-    def fields(self, m):
+    def fields(self, m=None):
         """
         Forward model the gravity field of the mesh on the receivers in the survey
 
@@ -160,7 +160,9 @@ def fields(self, m):
             Gravity fields generated by the given model on every receiver
             location.
         """
-        self.model = m
+        if m is not None:
+            self.model = m
+
         if self.store_sensitivities == "forward_only":
             # Compute the linear operation without forming the full dense G
             if self.engine == "choclo":
diff --git a/simpeg/potential_fields/magnetics/simulation.py b/simpeg/potential_fields/magnetics/simulation.py
index 610b933682..05449aded5 100644
--- a/simpeg/potential_fields/magnetics/simulation.py
+++ b/simpeg/potential_fields/magnetics/simulation.py
@@ -178,9 +178,10 @@ def M(self, M):
             M = np.asarray(M)
             self._M = M.reshape((self.nC, 3))
 
-    def fields(self, model):
-        self.model = model
-        # model = self.chiMap * model
+    def fields(self, m=None):
+        if m is not None:
+            self.model = m
+
         if self.store_sensitivities == "forward_only":
             if self.engine == "choclo":
                 fields = self._forward(self.chi)