diff --git a/simpeg_drivers-assets/uijson/joint_surveys_inversion.ui.json b/simpeg_drivers-assets/uijson/joint_surveys_inversion.ui.json
index e3e1b610..1ab64afa 100644
--- a/simpeg_drivers-assets/uijson/joint_surveys_inversion.ui.json
+++ b/simpeg_drivers-assets/uijson/joint_surveys_inversion.ui.json
@@ -76,7 +76,9 @@
         "main": true,
         "group": "Mesh and models",
         "label": "Model units",
-        "tooltip": "Select the units of the model",
+        "tooltip": "Change the model units between S/m or Ohm.m, for electromagnetic and direct-current surveys.",
+        "optional": true,
+        "enabled": false,
         "value": "Conductivity (S/m)"
     },
     "starting_model": {
diff --git a/simpeg_drivers/joint/joint_surveys/options.py b/simpeg_drivers/joint/joint_surveys/options.py
index edc0bbe9..a2dcb787 100644
--- a/simpeg_drivers/joint/joint_surveys/options.py
+++ b/simpeg_drivers/joint/joint_surveys/options.py
@@ -31,7 +31,7 @@ class JointSurveysModelOptions(JointModelOptions):
     :param reference_model: The reference model for the inversion.
     """
 
-    model_type: ModelTypeEnum = ModelTypeEnum.conductivity
+    model_type: ModelTypeEnum | None = ModelTypeEnum.conductivity
     starting_model: float | FloatData | None = None
     reference_model: float | FloatData | None = None
 
diff --git a/simpeg_drivers/options.py b/simpeg_drivers/options.py
index d086a4ea..ee9b78a7 100644
--- a/simpeg_drivers/options.py
+++ b/simpeg_drivers/options.py
@@ -335,7 +335,7 @@ class ConductivityModelOptions(ModelOptions):
     Options for the conductivity model used in all of EM methods.
     """
 
-    model_type: ModelTypeEnum = ModelTypeEnum.conductivity
+    model_type: ModelTypeEnum | None = ModelTypeEnum.conductivity
     conductivity_model: float | FloatData | None = Field(
         None,
         validation_alias=AliasChoices("background_conductivity", "conductivity_model"),
diff --git a/tests/run_tests/driver_dc_test.py b/tests/run_tests/driver_dc_test.py
index d0a4cdcd..72101839 100644
--- a/tests/run_tests/driver_dc_test.py
+++ b/tests/run_tests/driver_dc_test.py
@@ -78,6 +78,7 @@ def test_dc_3d_fwr_run(
             mesh=components.mesh,
             topography_object=components.topography,
             data_object=components.survey,
+            model_type=None,
             starting_model=components.model,
         )
     fwr_driver = DC3DForwardDriver(params)
@@ -104,6 +105,7 @@ def test_dc_3d_run(
             mesh=components.mesh,
             topography_object=components.topography,
             data_object=potential.parent,
+            model_type=None,
             starting_model=1e-2,
             reference_model=1e-2,
             s_norm=0.0,
diff --git a/tests/run_tests/driver_grav_test.py b/tests/run_tests/driver_grav_test.py
index 1aba275c..8f797614 100644
--- a/tests/run_tests/driver_grav_test.py
+++ b/tests/run_tests/driver_grav_test.py
@@ -68,6 +68,7 @@ def test_gravity_fwr_run(
             geoh5=geoh5,
             mesh=components.mesh,
             topography_object=components.topography,
+            model_type=None,
             data_object=components.survey,
             starting_model=components.model,
             gz_channel_bool=True,
@@ -142,6 +143,7 @@ def test_gravity_run(
         params = GravityInversionOptions.build(
             geoh5=geoh5,
             mesh=components.mesh,
+            model_type=None,
             data_object=gz.parent,
             s_norm=0.0,
             x_norm=gradient_norms,