diff --git a/src/core/kdtree.hpp b/src/core/kdtree.hpp
--- a/src/core/kdtree.hpp
+++ b/src/core/kdtree.hpp
@@ -274,7 +274,7 @@ class Kdtree {
 	}
 
 	template <typename Outputter>
-	void FindContainedRecursive(CoordT p1[2], CoordT p2[2], size_t node_idx, int level, Outputter outputter) const
+	void FindContainedRecursive(CoordT p1[2], CoordT p2[2], size_t node_idx, int level, const Outputter &outputter) const
 	{
 		/* Dimension index of current level */
 		int dim = level % 2;
@@ -458,7 +458,7 @@ public:
 	* @param outputter Callback used to return values from the search.
 	*/
 	template <typename Outputter>
-	void FindContained(CoordT x1, CoordT y1, CoordT x2, CoordT y2, Outputter outputter) const
+	void FindContained(CoordT x1, CoordT y1, CoordT x2, CoordT y2, const Outputter &outputter) const
 	{
 		assert(x1 < x2);
 		assert(y1 < y2);