diff --git a/otherlibs/site/test/run.t b/otherlibs/site/test/run.t
index 5ec0274697c..ee05d42a748 100644
--- a/otherlibs/site/test/run.t
+++ b/otherlibs/site/test/run.t
@@ -155,11 +155,6 @@ Test with an opam like installation
   ]
 
   $ dune build -p a @install
-  File "d/dune", line 5, characters 45-46:
-  5 | (generate_sites_module (module sites) (sites d))
-                                                   ^
-  Error: dune_site used outside a package
-  [1]
 
 
 
diff --git a/src/dune_rules/generate_sites_module_rules.ml b/src/dune_rules/generate_sites_module_rules.ml
index 9ee96afc833..295edc4bd4c 100644
--- a/src/dune_rules/generate_sites_module_rules.ml
+++ b/src/dune_rules/generate_sites_module_rules.ml
@@ -57,30 +57,35 @@ let plugins_code sctx buf pkg sites =
         (String.capitalize site) plugins plugins site)
 
 let setup_rules sctx ~dir (def : Dune_file.Generate_sites_module.t) =
-  let buf = Buffer.create 1024 in
-  if def.sourceroot then sourceroot_code buf;
-  if def.relocatable then relocatable_code buf;
-  let sites =
-    List.sort_uniq
-      ~compare:(fun (_, pkga) (_, pkgb) -> Package.Name.compare pkga pkgb)
-      (def.sites @ List.map ~f:(fun (loc, (pkg, _)) -> (loc, pkg)) def.plugins)
+  let impl () =
+    let buf = Buffer.create 1024 in
+    if def.sourceroot then sourceroot_code buf;
+    if def.relocatable then relocatable_code buf;
+    let sites =
+      List.sort_uniq
+        ~compare:(fun (_, pkga) (_, pkgb) -> Package.Name.compare pkga pkgb)
+        (def.sites @ List.map ~f:(fun (loc, (pkg, _)) -> (loc, pkg)) def.plugins)
+    in
+    if List.is_non_empty sites then (
+      pr buf "module Sites = struct";
+      List.iter sites ~f:(sites_code sctx buf);
+      pr buf "end"
+    );
+    let plugins =
+      Package.Name.Map.of_list_multi (List.map ~f:snd def.plugins)
+    in
+    if not (Package.Name.Map.is_empty plugins) then (
+      pr buf "module Plugins = struct";
+      Package.Name.Map.iteri plugins ~f:(plugins_code sctx buf);
+      pr buf "end"
+    );
+    Buffer.contents buf
   in
-  if List.is_non_empty sites then (
-    pr buf "module Sites = struct";
-    List.iter sites ~f:(sites_code sctx buf);
-    pr buf "end"
-  );
-  let plugins = Package.Name.Map.of_list_multi (List.map ~f:snd def.plugins) in
-  if not (Package.Name.Map.is_empty plugins) then (
-    pr buf "module Plugins = struct";
-    Package.Name.Map.iteri plugins ~f:(plugins_code sctx buf);
-    pr buf "end"
-  );
-  let impl = Buffer.contents buf in
   let module_ = Module_name.to_string def.module_ ^ ".ml" in
   let file = Path.Build.relative dir module_ in
   let open Memo.Build.O in
   let+ () =
-    Super_context.add_rule sctx ~dir (Action_builder.write_file file impl)
+    Super_context.add_rule sctx ~dir
+      (Action_builder.write_file_dyn file (Action_builder.delayed impl))
   in
   module_