diff --git a/net/LWIPInterface/lwip-sys/arch/sys_arch.c b/net/LWIPInterface/lwip-sys/arch/sys_arch.c
index 8eead45b27..25e2f14ef5 100644
--- a/net/LWIPInterface/lwip-sys/arch/sys_arch.c
+++ b/net/LWIPInterface/lwip-sys/arch/sys_arch.c
@@ -285,7 +285,7 @@ void sys_sem_free(sys_sem_t *sem) {}
  * @return a new mutex */
 err_t sys_mutex_new(sys_mutex_t *mutex) {
 #ifdef CMSIS_OS_RTX
-#ifdef __MBED_CMSIS_RTOS_CA9
+#if defined(__MBED_CMSIS_RTOS_CA9) || defined(__MBED_CMSIS_RTOS_CM)
     memset(mutex->data, 0, sizeof(int32_t)*4);
 #else
     memset(mutex->data, 0, sizeof(int32_t)*3);
diff --git a/net/LWIPInterface/lwip-sys/arch/sys_arch.h b/net/LWIPInterface/lwip-sys/arch/sys_arch.h
index 620b734b30..7678e24eab 100644
--- a/net/LWIPInterface/lwip-sys/arch/sys_arch.h
+++ b/net/LWIPInterface/lwip-sys/arch/sys_arch.h
@@ -40,7 +40,7 @@ typedef struct {
     osMutexId    id;
     osMutexDef_t def;
 #ifdef CMSIS_OS_RTX
-#ifdef __MBED_CMSIS_RTOS_CA9
+#if defined(__MBED_CMSIS_RTOS_CA9) || defined(__MBED_CMSIS_RTOS_CM)
     int32_t      data[4];
 #else
     int32_t      data[3];