diff --git a/docs/src/config/boundaries.md b/docs/src/config/boundaries.md
index c07a5c412..0fd54076d 100644
--- a/docs/src/config/boundaries.md
+++ b/docs/src/config/boundaries.md
@@ -130,7 +130,8 @@ The available options are:
 
   - `"SmallerRefractiveIndex"` :  Take the value from the side where the material index of
     refraction is smaller (speed of light is larger). Typically this selects the vacuum
-    side.
+    side. For anisotropic materials, the index of refraction associated with the principal
+    direction with the smallest value is used.
   - `"LargerRefractiveIndex"` :  Take the value from the side where the material index of
     refraction is larger (speed of light is smaller). Typically this selects the non-vacuum
     side.
@@ -609,7 +610,8 @@ general double-valued). The available options are:
 
   - `"SmallerRefractiveIndex"` :  Take the value from the side where the material index of
     refraction is smaller (speed of light is larger). Typically this selects the vacuum
-    side.
+    side. For anisotropic materials, the index of refraction associated with the principal
+    direction with the smallest value is used.
   - `"LargerRefractiveIndex"` :  Take the value from the side where the material index of
     refraction is larger (speed of light is smaller). Typically this selects the non-vacuum
     side.
diff --git a/palace/fem/coefficient.hpp b/palace/fem/coefficient.hpp
index 27f620b00..85fdf4e98 100644
--- a/palace/fem/coefficient.hpp
+++ b/palace/fem/coefficient.hpp
@@ -49,7 +49,7 @@ class BdrGridFunctionCoefficient
   {
     // For interior faces with no way to distinguish on which side to evaluate quantities,
     // evaluate on both (using the average or sum, depending on the application).
-    return (FET.Elem2 && mat_op.GetLightSpeedMin(FET.Elem2->Attribute) ==
+    return (FET.Elem2 && mat_op.GetLightSpeedMax(FET.Elem2->Attribute) ==
                              mat_op.GetLightSpeedMax(FET.Elem1->Attribute));
   }
 
@@ -60,7 +60,7 @@ class BdrGridFunctionCoefficient
     // (refractive index is smaller, typically should choose the vacuum side). For cases
     // where the speeds are the same, use element 1.
     return (FET.Elem2 && side_n_min &&
-            mat_op.GetLightSpeedMin(FET.Elem2->Attribute) >
+            mat_op.GetLightSpeedMax(FET.Elem2->Attribute) >
                 mat_op.GetLightSpeedMax(FET.Elem1->Attribute));
   }