diff --git a/cyassl/ctaocrypt/port.h b/cyassl/ctaocrypt/port.h
index 95a432304..1591d4c8b 100644
--- a/cyassl/ctaocrypt/port.h
+++ b/cyassl/ctaocrypt/port.h
@@ -95,7 +95,11 @@
     #elif defined(FREESCALE_MQX)
         typedef MUTEX_STRUCT CyaSSL_Mutex;
     #elif defined(CYASSL_MDK_ARM)
-        typedef OS_MUT CyaSSL_Mutex;
+        #if defined(CYASSL_CMSIS_RTOS)
+            typedef osMutexId CyaSSL_Mutex;
+        #else
+            typedef OS_MUT CyaSSL_Mutex;
+        #endif
     #else
         #error Need a mutex type in multithreaded mode
     #endif /* USE_WINDOWS_API */