diff --git a/src/libutil/finally.hh b/src/libutil/finally.hh
index 47c64deae..7760cfe9a 100644
--- a/src/libutil/finally.hh
+++ b/src/libutil/finally.hh
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <functional>
+
 /* A trivial class to run a function at the end of a scope. */
 class Finally
 {