diff --git a/src/libutil/finally.hh b/src/libutil/finally.hh
index 47c64deaecea05af5b6e5f519124e747f2916918..7760cfe9a4104353b921f9d5356170d3debeacdb 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
 {