diff --git a/rtos/Mutex.h b/rtos/Mutex.h
index 4c738b7e7b..1b2b106bc4 100644
--- a/rtos/Mutex.h
+++ b/rtos/Mutex.h
@@ -29,6 +29,7 @@
 
 #include "platform/NonCopyable.h"
 #include "platform/ScopedLock.h"
+#include "platform/mbed_toolchain.h"
 
 namespace rtos {
 /** \addtogroup rtos */