diff --git a/dumux/common/boundingboxtree.hh b/dumux/common/boundingboxtree.hh
index 74a890739a76c3fb7dc87c3175985c288c8082c0..87cda092bec94522f73b673eb4272b77a48d8c38 100644
--- a/dumux/common/boundingboxtree.hh
+++ b/dumux/common/boundingboxtree.hh
@@ -758,6 +758,10 @@ private:
 template <class GridView>
 class BoundingBoxTree
 {
+    // be friends with all other kinds bounding box trees so that
+    // they can call each others private methods
+    template <class OtherGridView> friend class BoundingBoxTree;
+
     static const int dim = GridView::dimension;
     static const int dimworld = GridView::dimensionworld;
     typedef typename GridView::template Codim<0>::Entity Element;