diff --git a/src/build.ml b/src/build.ml
index 6d41d3b7..b77c48b1 100644
--- a/src/build.ml
+++ b/src/build.ml
@@ -131,6 +131,18 @@ let dyn_paths t = Dyn_paths t
 let contents p = Contents p
 let lines_of p = Lines_of p
 
+let read_sexp p =
+  contents p
+  >>^ fun s ->
+  let lb = Lexing.from_string s in
+  lb.lex_curr_p <-
+    { pos_fname = Path.to_string p
+    ; pos_lnum  = 1
+    ; pos_bol   = 0
+    ; pos_cnum  = 0
+    };
+  Sexp_lexer.single lb
+
 let if_file_exists p ~then_ ~else_ =
   If_file_exists (p, ref (Undecided (then_, else_)))
 
diff --git a/src/build.mli b/src/build.mli
index 197f9e4e..36e88a8f 100644
--- a/src/build.mli
+++ b/src/build.mli
@@ -45,6 +45,9 @@ val dyn_paths : ('a, Path.t list) t -> ('a, 'a) t
 val contents : Path.t -> ('a, string) t
 val lines_of : Path.t -> ('a, string list) t
 
+(** Load an S-expression from a file *)
+val read_sexp : Path.t -> (unit, Sexp.Ast.t) t
+
 (** Evaluates to [true] if the file is present on the file system or is the target of a
     rule. *)
 val file_exists : Path.t -> ('a, bool)  t
diff --git a/src/super_context.ml b/src/super_context.ml
index e502dbde..0b068c28 100644
--- a/src/super_context.ml
+++ b/src/super_context.ml
@@ -720,13 +720,8 @@ let expand_and_eval_set ~dir set ~standard =
     Build.return (Ordered_set_lang.eval_with_standard set ~standard)
   | files ->
     let paths = List.map files ~f:(Path.relative dir) in
-    Build.paths paths
-    >>>
-    Build.arr (fun () ->
-      let files_contents =
-        List.map2 files paths ~f:(fun fn path ->
-          (fn, Sexp_load.single (Path.to_string path)))
-        |> String_map.of_alist_exn
-      in
-      let set = Ordered_set_lang.Unexpanded.expand set ~files_contents in
-      Ordered_set_lang.eval_with_standard set ~standard)
+    Build.all (List.map paths ~f:Build.read_sexp)
+    >>^ fun sexps ->
+    let files_contents = List.combine files sexps |> String_map.of_alist_exn in
+    let set = Ordered_set_lang.Unexpanded.expand set ~files_contents in
+    Ordered_set_lang.eval_with_standard set ~standard