diff --git a/src/Plankton/PlanktonVertexList.cs b/src/Plankton/PlanktonVertexList.cs
index cb056be..3d9c52e 100644
--- a/src/Plankton/PlanktonVertexList.cs
+++ b/src/Plankton/PlanktonVertexList.cs
@@ -395,8 +395,8 @@ public PlanktonXYZ GetNormal(int index)
             if (this.IsBoundary(index) == false)
             {
                 normal += PlanktonXYZ.CrossProduct(
-                    this[n-1].ToXYZ() - vertex,
-                    this[0].ToXYZ() - vertex);
+                    this[ring[n-1]].ToXYZ() - vertex,
+                    this[ring[0]].ToXYZ() - vertex);
             }
 
             return normal * (-1.0f / normal.Length); // return unit vector