diff --git a/src/SequentialPicker.h b/src/SequentialPicker.h
index eb4489b0..d03ed45b 100644
--- a/src/SequentialPicker.h
+++ b/src/SequentialPicker.h
@@ -39,6 +39,7 @@
 
 #include <deque>
 #include <memory>
+#include <functional>
 
 namespace aria2 {