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/tests/run_tests/driver_joint_surveys_test.py b/tests/run_tests/driver_joint_surveys_test.py
index dd6b26fd..d0b69a6d 100644
--- a/tests/run_tests/driver_joint_surveys_test.py
+++ b/tests/run_tests/driver_joint_surveys_test.py
@@ -403,6 +403,7 @@ def test_joint_surveys_tem_run(
             mesh=components.mesh,
             group_a=driver_A.out_group,
             group_b=driver_B.out_group,
+            model_type=None,
             starting_model=1e-3,
         )