diff --git a/CHANGES b/CHANGES
index a6d415fa7..096e1c1fd 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,9 @@
+===== 6.1.2 =====
+
+====== Fixes ======
+
+  * fix initialisation of lock on Windows. (Mark Elvers, David Allsopp, #1103, #1107, #1104)
+
 ===== 6.1.1 =====
 
 ====== Fixes ======
diff --git a/src/unix/lwt_unix_stubs.c b/src/unix/lwt_unix_stubs.c
index d77237a4c..83a21a2e4 100644
--- a/src/unix/lwt_unix_stubs.c
+++ b/src/unix/lwt_unix_stubs.c
@@ -1063,6 +1063,13 @@ CAMLprim value lwt_unix_start_job(value val_job, value val_async_method) {
   lwt_unix_async_method async_method = Int_val(val_async_method);
   int done = 0;
 
+  /* Ensure threading is initialized before using pool_mutex.
+     On Windows, CRITICAL_SECTION must be initialized before use;
+     unlike pthreads, zero-initialized CRITICAL_SECTIONs are invalid. */
+  if (async_method != LWT_UNIX_ASYNC_METHOD_NONE) {
+    initialize_threading();
+  }
+
   /* Fallback to synchronous call if there is no worker available and
      we can not launch more threads. */
   if (async_method != LWT_UNIX_ASYNC_METHOD_NONE) {
@@ -1087,8 +1094,6 @@ CAMLprim value lwt_unix_start_job(value val_job, value val_async_method) {
 
     case LWT_UNIX_ASYNC_METHOD_DETACH:
     case LWT_UNIX_ASYNC_METHOD_SWITCH:
-      initialize_threading();
-
       lwt_unix_mutex_init(&job->mutex);
 
       lwt_unix_mutex_lock(&pool_mutex);