diff --git a/common/os/Thread.h b/common/os/Thread.h
index bca41bb..4c39884 100644
--- a/common/os/Thread.h
+++ b/common/os/Thread.h
@@ -19,6 +19,8 @@
 #ifndef __OS_THREAD_H__
 #define __OS_THREAD_H__
 
+#include <stddef.h>
+
 namespace os {
   class Mutex;