diff --git a/timer.hpp b/timer.hpp
index e6b0693..058740e 100644
--- a/timer.hpp
+++ b/timer.hpp
@@ -1,5 +1,6 @@
 #pragma once
 
+#include <functional>
 #include <memory>
 #include <chrono>
 #include <systemd/sd-event.h>