diff --git a/CHANGES.md b/CHANGES.md
index a55b558ab3f..c29ba1c47a6 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -13,6 +13,8 @@ Unreleased
 - Allow `$ dune ocaml dump-dot-merlin` to run in watch mode. Also this command
   shouldn't print "Entering Directory" mesages. (#6497, @rgrinberg)
 
+- Fix build with MSVC compiler (#6517, @nojb)
+
 3.6.0 (2022-11-14)
 ------------------
 
diff --git a/src/dune_util/dune_flock.c b/src/dune_util/dune_flock.c
index cd338848e8c..13832f1d4cd 100644
--- a/src/dune_util/dune_flock.c
+++ b/src/dune_util/dune_flock.c
@@ -12,6 +12,7 @@
 CAMLprim value dune_flock_lock(value v_fd, value v_block, value v_exclusive) {
 #ifdef _WIN32
   caml_failwith("no flock on win32");
+  return Val_unit;
 #else
   CAMLparam2(v_fd, v_block);
   int flags = 0;
@@ -35,6 +36,7 @@ CAMLprim value dune_flock_lock(value v_fd, value v_block, value v_exclusive) {
 CAMLprim value dune_flock_unlock(value v_fd) {
 #ifdef _WIN32
   caml_failwith("no flock on win32");
+  return Val_unit;
 #else
   CAMLparam1(v_fd);
   caml_release_runtime_system();