diff --git a/docs/api/fdtd.md b/docs/api/fdtd.md
index f3e7ac2..2ceaf56 100644
--- a/docs/api/fdtd.md
+++ b/docs/api/fdtd.md
@@ -18,3 +18,7 @@ CPML derivatives use electric widths for forward differences and magnetic
 widths for backward differences. `cpml_params(..., dx=1.0)` specifies the uniform
 physical cell width normal to a face; arbitrary nonuniform PML grading is not
 implemented by this constructor. Scale the timestep consistently with the grid.
+
+`ln_R_per_layer` must be finite and strictly negative. Zero attenuation raises
+`ValueError`; disable individual CPML faces with `None` in the parameter table
+passed to `updates_with_cpml`.
diff --git a/meanas/fdtd/pml.py b/meanas/fdtd/pml.py
index 679bb25..976368a 100644
--- a/meanas/fdtd/pml.py
+++ b/meanas/fdtd/pml.py
@@ -52,7 +52,9 @@ def cpml_params(
             `+1` for the high-index face).
         dt: Timestep used by the Yee update.
         thickness: Number of Yee cells occupied by the CPML region.
-        ln_R_per_layer: Logarithmic attenuation target per layer.
+        ln_R_per_layer: Finite, strictly negative logarithmic attenuation target
+            per layer. Zero attenuation is not supported; disable a face with
+            ``None`` in the updates_with_cpml parameter table instead.
         epsilon_eff: Effective permittivity used when choosing the CPML scaling.
         mu_eff: Effective permeability used when choosing the CPML scaling.
         m: Polynomial grading exponent for `sigma` and `kappa`.
@@ -82,6 +84,9 @@ def cpml_params(
     if epsilon_eff <= 0:
         raise ValueError('epsilon_eff must be positive')
 
+    if not numpy.isfinite(ln_R_per_layer) or ln_R_per_layer >= 0:
+        raise ValueError('ln_R_per_layer must be finite and negative; use None to disable a CPML face')
+
     if not numpy.isfinite(dx) or dx <= 0:
         raise ValueError('dx must be finite and positive')
 
diff --git a/meanas/test/test_numerical_feedback.py b/meanas/test/test_numerical_feedback.py
index 7c15091..1658bb7 100644
--- a/meanas/test/test_numerical_feedback.py
+++ b/meanas/test/test_numerical_feedback.py
@@ -400,3 +400,10 @@ def test_scpml_builders_agree_including_real_profiles_and_touching_faces(shape,
     for family in range(2):
         for axis in range(3):
             assert_allclose(uniform[family][axis], explicit[family][axis], rtol=1e-14, atol=1e-14)
+
+
+@pytest.mark.parametrize('attenuation', [0.0, -0.0, .1, np.inf, -np.inf, np.nan])
+@pytest.mark.parametrize('alpha', [0, .2])
+def test_cpml_rejects_zero_or_invalid_attenuation_before_coefficient_evaluation(attenuation, alpha):
+    with np.errstate(divide='raise', invalid='raise'), pytest.raises(ValueError, match='ln_R_per_layer must be finite and negative'):
+        pml.cpml_params(0, 1, .1, ln_R_per_layer=attenuation, cfs_alpha=alpha)