diff --git a/src/harmonica/_transformations.py b/src/harmonica/_transformations.py
index 0e16cb6c5..6387a0da0 100644
--- a/src/harmonica/_transformations.py
+++ b/src/harmonica/_transformations.py
@@ -389,8 +389,8 @@ def reduction_to_pole(
     grid,
     inclination,
     declination,
-    magnetization_inclination=None,
-    magnetization_declination=None,
+    magnetization_inclination,
+    magnetization_declination,
     *,
     pad=True,
     pad_kwargs=None,
@@ -412,16 +412,10 @@ def reduction_to_pole(
         The inclination of the inducing Geomagnetic field.
     declination : float in degrees
         The declination of the inducing Geomagnetic field.
-    magnetization_inclination : float in degrees or None
-        The inclination of the total magnetization of the anomaly source. If
-        None, the ``magnetization_inclination`` will be set equal to the
-        ``inclination``, neglecting remanent magnetization and self
-        demagnetization. Default None.
+    magnetization_inclination : float in degrees
+        The inclination of the total magnetization of the anomaly source.
     magnetization_declination : float in degrees
-        The declination of the total magnetization of the anomaly source. If
-        None, the ``magnetization_declination`` will be set equal to the
-        ``declination``, neglecting remanent magnetization and self
-        demagnetization. Default None.
+        The declination of the total magnetization of the anomaly source.
     pad : bool, optional
         If True, will add padding to the grid before taking the Fourier Transform
         and applying the filter and remove it after the inverse Fourier Transform.
diff --git a/test/test_transformations.py b/test/test_transformations.py
index 19209f6f3..0d7954b4d 100644
--- a/test/test_transformations.py
+++ b/test/test_transformations.py
@@ -553,6 +553,21 @@ def test_reduction_to_pole_dim_names(sample_potential):
     reduction_to_pole(renamed_dims_grid, 60, 45, 60, 45)
 
 
+def test_reduction_to_pole_magnetization_required(sample_potential):
+    """
+    The magnetization direction must be a required argument.
+
+    Making it required forces users to be explicit about the induced-only
+    magnetization assumption (see #440 and #661). It used to default to None on
+    the public ``reduction_to_pole`` function, which crashed deep inside the
+    kernel after #661 removed the None handling.
+    """
+    with pytest.raises(TypeError, match="magnetization_inclination"):
+        reduction_to_pole(sample_potential, 60, 45)
+    with pytest.raises(TypeError, match="magnetization_declination"):
+        reduction_to_pole(sample_potential, 60, 45, 60)
+
+
 class TestTotalGradientAmplitude:
     """
     Test total_gradient_amplitude function.