diff --git a/test/flat_tree_test.cpp b/test/flat_tree_test.cpp
index e94c089..e65b73a 100644
--- a/test/flat_tree_test.cpp
+++ b/test/flat_tree_test.cpp
@@ -16,6 +16,7 @@
 
 #include "movable_int.hpp"
 #include "dummy_test_allocator.hpp"
+#include <functional> //std::less
 
 using namespace boost::container;