diff --git a/src/invert4geom/regional.py b/src/invert4geom/regional.py
index 499c78d4..e8c11a2b 100644
--- a/src/invert4geom/regional.py
+++ b/src/invert4geom/regional.py
@@ -482,6 +482,8 @@ def regional_constraints(
                 spline_damping_values.append(None)
         else:
             spline_damping_values = spline_damping
+        if spline_damping_values is None:
+            spline_damping_values = [None]
         spline = utils.best_spline_cv(
             coordinates=(
                 constraints_df.easting,
diff --git a/tests/test_regional.py b/tests/test_regional.py
index e9e9b258..ac64b509 100644
--- a/tests/test_regional.py
+++ b/tests/test_regional.py
@@ -212,6 +212,7 @@ def test_regional_constraints(test_input):
         grid_method=test_input,
         grav_obs_height=1e3,
         source_depth=100e3,
+        spline_damping=1e-3,
     )
 
     # grid = df.set_index(["northing", "easting"]).to_xarray()