diff --git a/tools/ci_setup.ml b/tools/ci_setup.ml
index fd8bcaa87..aae625ea1 100644
--- a/tools/ci_setup.ml
+++ b/tools/ci_setup.ml
@@ -245,7 +245,7 @@ let clone' delay ?branch ?commit nm src =
         (Printf.sprintf "cd jane-street/lib/%s && git checkout %s" nm commit)
 
 let () =
-  let ch = open_bin "jane-street/dune-workspace" in
+  let ch = open_in_bin "jane-street/dune-workspace" in
   output_string ch dune_workspace;
   close_out ch