--- a/third_party/proton/csrc/include/Utility/Map.h
+++ b/third_party/proton/csrc/include/Utility/Map.h
@@ -1,6 +1,7 @@
 #ifndef PROTON_UTILITY_MAP_H_
 #define PROTON_UTILITY_MAP_H_
 
+#include <functional>
 #include <map>
 #include <mutex>
 #include <shared_mutex>