diff --git a/src/DHTConnectionSocksProxyImpl.cc b/src/DHTConnectionSocksProxyImpl.cc
index 852b094f..4684e828 100644
--- a/src/DHTConnectionSocksProxyImpl.cc
+++ b/src/DHTConnectionSocksProxyImpl.cc
@@ -38,13 +38,14 @@
 #include <cstring>
 
 #include "SocketCore.h"
+#include "a2functional.h"
 
 namespace aria2 {
 
 DHTConnectionSocksProxyImpl::DHTConnectionSocksProxyImpl(int family)
     : DHTConnectionImpl(family),
       family_(family),
-      proxySocket_(std::make_unique<SocketCore>())
+      proxySocket_(make_unique<SocketCore>())
 {
 }