diff --git a/src/invert4geom/cross_validation.py b/src/invert4geom/cross_validation.py
index adfc5c12..153d7177 100644
--- a/src/invert4geom/cross_validation.py
+++ b/src/invert4geom/cross_validation.py
@@ -540,13 +540,13 @@ def zref_density_optimal_parameter(
         raise ValueError(msg)
 
     if zref_values is None:
-        zref = kwargs.get("zref", None)
+        zref = kwargs.get("zref")
         if zref is None:
             msg = "must provide zref_values or zref in kwargs"
             raise ValueError(msg)
         zref_values = [zref]
     elif density_contrast_values is None:
-        density_contrast = kwargs.get("density_contrast", None)
+        density_contrast = kwargs.get("density_contrast")
         if density_contrast is None:
             msg = "must provide density_contrast_values or density_contrast in kwargs"
             raise ValueError(msg)
diff --git a/src/invert4geom/plotting.py b/src/invert4geom/plotting.py
index 26e10267..a55ebda1 100644
--- a/src/invert4geom/plotting.py
+++ b/src/invert4geom/plotting.py
@@ -1104,7 +1104,7 @@ def show_prism_layers(
         notebook=True,
     )
 
-    opacity = kwargs.get("opacity", None)
+    opacity = kwargs.get("opacity")
 
     if isinstance(prisms, xr.Dataset):
         prisms = [prisms]
@@ -1126,7 +1126,7 @@ def show_prism_layers(
                 style=kwargs.get("style", "surface"),
                 show_edges=kwargs.get("show_edges", False),
                 opacity=trans,
-                scalar_bar_args=kwargs.get("scalar_bar_args", None),
+                scalar_bar_args=kwargs.get("scalar_bar_args"),
             )
         else:
             plotter.add_mesh(
@@ -1139,7 +1139,7 @@ def show_prism_layers(
                 show_edges=kwargs.get("show_edges", False),
                 log_scale=kwargs.get("log_scale", True),
                 opacity=trans,
-                scalar_bar_args=kwargs.get("scalar_bar_args", None),
+                scalar_bar_args=kwargs.get("scalar_bar_args"),
             )
         plotter.set_scale(
             zscale=kwargs.get("zscale", 75)
@@ -1336,7 +1336,7 @@ def plot_stochastic_results(
     unit = kwargs.get("unit", "m")
     reverse_cpt = kwargs.get("reverse_cpt", True)
     label = kwargs.get("label", "ensemble mean")
-    points_label = kwargs.get("points_label", None)
+    points_label = kwargs.get("points_label")
     fig_height = kwargs.get("fig_height", 12)
 
     try:
diff --git a/tests/test_inversion.py b/tests/test_inversion.py
index e1c307e4..55c1aa7e 100644
--- a/tests/test_inversion.py
+++ b/tests/test_inversion.py
@@ -171,7 +171,7 @@ def dummy_jacobian_square() -> NDArray:
 ################
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 def test_grav_column_der_relative_values():
     """
     test the grav_column_der function
@@ -275,7 +275,7 @@ def test_grav_column_der_relative_values():
     assert h < a
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 def test_grav_column_der():
     """
     test the grav_column_der function against a small prism approximation
@@ -306,7 +306,7 @@ def test_grav_column_der():
     assert dg_z == pytest.approx(hm_dg_z, rel=1e-2)
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 def test_jacobian_annular():
     """
     test the jacobian_annular function
@@ -359,7 +359,7 @@ def test_prism_properties_error():
         inversion._prism_properties(prisms_layer, method="wrong_input")
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 def test_jacobian_prism():
     """
     test the jacobian_prism function
@@ -385,7 +385,7 @@ def test_jacobian_prism():
     assert jac[:, 0:-3].min() > 0
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 def test_jacobian():
     """
     test the jacobian dispatcher function
@@ -470,7 +470,7 @@ def test_solver_square_error():
 ]
 
 
-@pytest.mark.use_numba()
+@pytest.mark.use_numba
 @pytest.mark.parametrize("solver_type", solver_types)
 def test_solver_square(solver_type):
     """