diff --git a/src/ProxyServer.hh b/src/ProxyServer.hh
index 2e505f60..e510b466 100644
--- a/src/ProxyServer.hh
+++ b/src/ProxyServer.hh
@@ -5,6 +5,7 @@
 #include <map>
 #include <unordered_map>
 #include <unordered_set>
+#include <functional>
 #include <vector>
 #include <string>
 #include <memory>