diff --git a/bin/fiber/fiber.ml b/bin/fiber/fiber.ml
index 188a92cc9..6087a8687 100644
--- a/bin/fiber/fiber.ml
+++ b/bin/fiber/fiber.ml
@@ -129,7 +129,7 @@ let worker pool =
 
 let get_concurrency () =
   try
-    let ic = Unix.open_process_in "getconf _NPROCESSORS_ONLN" in
+    let ic = Unix.open_process_in "@getconf@/bin/getconf _NPROCESSORS_ONLN" in
     let close () = ignore (Unix.close_process_in ic) in
     let sc = Scanf.Scanning.from_channel ic in
     try