diff --git a/collective_effects/resistive_wall.py b/collective_effects/resistive_wall.py
index eb5b1982360eab97fe3fa3749512bde1ed886514..1bb385419d6324cd274945b6194f4cb3b0d513f2 100644
--- a/collective_effects/resistive_wall.py
+++ b/collective_effects/resistive_wall.py
@@ -304,7 +304,7 @@ class Coating(WakeField):
             bracket = ( (np.sign(f) - 1j) * skin1 * 
                        (alpha * tanh + 1) / (alpha + tanh) )
         else:
-            valid_approx = self.thickness / skin1
+            valid_approx = self.thickness / np.min(skin1)
             if valid_approx < 0.01:
                 print("Approximation is not valid. Returning impedance anyway.")
             bracket = ( (np.sign(f) - 1j) * skin2 - 2 * 1j * self.thickness * 
@@ -352,7 +352,7 @@ class Coating(WakeField):
             bracket = ( (1 - 1j*np.sign(f)) * skin1 * 
                        (alpha * tanh + 1) / (alpha + tanh) )
         else:
-            valid_approx = self.thickness / skin1
+            valid_approx = self.thickness / np.min(skin1)
             if valid_approx < 0.01:
                 print("Approximation is not valid. Returning impedance anyway.")
             bracket = ( (1 - 1j*np.sign(f)) * skin2 - 2 * 1j * self.thickness