diff --git a/kdtree++/kdtree.hpp b/kdtree++/kdtree.hpp
index 53bbfe4..665ba21 100644
--- a/kdtree++/kdtree.hpp
+++ b/kdtree++/kdtree.hpp
@@ -67,6 +67,7 @@
 #  include <map>
 #endif
 #include <algorithm>
+#include <functional>
 
 #ifdef KDTREE_DEFINE_OSTREAM_OPERATORS
 #  include <ostream>