diff --git a/src/dune_util/dune_flock.c b/src/dune_util/dune_flock.c
index fefd342c84f..badf45374d6 100644
--- a/src/dune_util/dune_flock.c
+++ b/src/dune_util/dune_flock.c
@@ -52,6 +52,8 @@ CAMLprim value dune_flock_lock(value v_fd, value v_block, value v_exclusive) {
   int flags = 0;
   if (Bool_val(v_exclusive)) {
     flags |= LOCK_EX;
+  } else {
+    flags |= LOCK_SH;
   }
   if (!Bool_val(v_block)) {
     flags |= LOCK_NB;