diff --git a/src/stl_logging_unittest.cc b/src/stl_logging_unittest.cc
index 63e28a1..89fd6a1 100644
--- a/src/stl_logging_unittest.cc
+++ b/src/stl_logging_unittest.cc
@@ -31,6 +31,7 @@
 
 #ifdef HAVE_USING_OPERATOR
 
+#include <functional>
 #include <iostream>
 #include <map>
 #include <ostream>