diff --git a/dlib/threads/async.cpp b/dlib/threads/async.cpp
index b6b295651..6aa947bcb 100644
--- a/dlib/threads/async.cpp
+++ b/dlib/threads/async.cpp
@@ -9,6 +9,7 @@
 #include "async.h"
 #include <stdlib.h>
 #include "../string.h"
+#include <thread>
 
 namespace dlib
 {