diff --git a/unsupported/doc/examples/BVH_Example.cpp b/unsupported/doc/examples/BVH_Example.cpp
index 9e9959a32..6b6fac075 100644
--- a/unsupported/doc/examples/BVH_Example.cpp
+++ b/unsupported/doc/examples/BVH_Example.cpp
@@ -5,7 +5,11 @@
 using namespace Eigen;
 typedef AlignedBox<double, 2> Box2d;
 
-Box2d internal::bounding_box(const Vector2d &v) { return Box2d(v, v); } //compute the bounding box of a single point
+namespace Eigen {
+    namespace internal {
+        Box2d bounding_box(const Vector2d &v) { return Box2d(v, v); } //compute the bounding box of a single point
+    }
+}
 
 struct PointPointMinimizer //how to compute squared distances between points and rectangles
 {