diff --git a/util/string_piece_hash.hh b/util/string_piece_hash.hh
index 6ff6f88..16808cd 100644
--- a/util/string_piece_hash.hh
+++ b/util/string_piece_hash.hh
@@ -1,6 +1,7 @@
 #ifndef UTIL_STRING_PIECE_HASH_H
 #define UTIL_STRING_PIECE_HASH_H
 
+#include <functional>
 #include "util/have.hh"
 #include "util/string_piece.hh"