diff --git a/src/libstore/nar-accessor.hh b/src/libstore/nar-accessor.hh
index 1903355a..2871199d 100644
--- a/src/libstore/nar-accessor.hh
+++ b/src/libstore/nar-accessor.hh
@@ -1,5 +1,7 @@
 #pragma once
 
+#include <functional>
+
 #include "fs-accessor.hh"
 
 namespace nix {