diff --git a/src/kernel_services/ast_queries/file.ml b/src/kernel_services/ast_queries/file.ml
index 8bdc821d652bdf22e95fcdc330084d1d1b8c8771..8ba8366645adec9b23700e0b4033f7e8a8328af0 100644
--- a/src/kernel_services/ast_queries/file.ml
+++ b/src/kernel_services/ast_queries/file.ml
@@ -139,7 +139,7 @@ let from_filename ?cpp f =
         | Some cpp -> cpp, cpp_opt_kind ()
       in
       let flags = if extra_flags <> "" then [extra_flags] else jcdb_flags in
-      (if flags = [] then cpp else cpp ^ " " ^ String.concat " " flags), gnu
+      String.concat " " (cpp :: flags), gnu
   in
   if Filename.check_suffix (f:>string) ".i" then begin
     NoCPP f