--- a/gfx/layers/BSPTree.cpp
+++ b/gfx/layers/BSPTree.cpp
@@ -58,27 +58,28 @@ BSPTree::CalculateDotProduct(const gfx::
   const float epsilon = 0.05f;
 
   const gfx::Point3D& normal = aFirst.GetNormal();
   const gfx::Point3D& planePoint = aFirst[0];
 
   nsTArray<float> dotProducts;
 
   for (const gfx::Point3D& point : aSecond.GetPoints()) {
-    const float dot = (point - planePoint).DotProduct(normal);
-    dotProducts.AppendElement(dot);
+    float dot = (point - planePoint).DotProduct(normal);
 
     if (dot > epsilon) {
       aPos++;
     } else if (dot < -epsilon) {
       aNeg++;
     } else {
       // The point is within the thick plane.
-      dotProducts[dotProducts.Length() - 1] = 0.0f;
+      dot = 0.0f;
     }
+
+    dotProducts.AppendElement(dot);
   }
 
   return dotProducts;
 }
 
 void
 BSPTree::BuildTree(UniquePtr<BSPTreeNode>& aRoot,
                    std::deque<gfx::Polygon3D>& aPolygons)