diff --git a/src/gc/os_dep.c b/src/gc/os_dep.c
index 21d05635a..ad2b07dca 100644
--- a/src/gc/os_dep.c
+++ b/src/gc/os_dep.c
@@ -3505,8 +3505,10 @@ static void *GC_mprotect_thread(void *arg) {
 
     mach_msg_id_t id;
 
+#if defined(THREADS)
     GC_darwin_register_mach_handler_thread(mach_thread_self());
-    
+#endif
+
     for(;;) {
         r = mach_msg(
             &msg.head,