diff --git a/src/cxxutil.hpp b/src/cxxutil.hpp
index 56b85c79b..835d5e74f 100644
--- a/src/cxxutil.hpp
+++ b/src/cxxutil.hpp
@@ -2,6 +2,7 @@
 #define __SRB2_CXXUTIL_HPP__
 
 #include <cstdlib>
+#include <functional>
 #include <type_traits>
 #include <utility>