diff --git a/libraries/net/lwip/lwip-sys/arch/sys_arch.c b/libraries/net/lwip/lwip-sys/arch/sys_arch.c
index 78f9827aa3..8eead45b27 100644
--- a/libraries/net/lwip/lwip-sys/arch/sys_arch.c
+++ b/libraries/net/lwip/lwip-sys/arch/sys_arch.c
@@ -285,7 +285,11 @@ 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
+    memset(mutex->data, 0, sizeof(int32_t)*4);
+#else
     memset(mutex->data, 0, sizeof(int32_t)*3);
+#endif
     mutex->def.mutex = mutex->data;
 #endif
     mutex->id = osMutexCreate(&mutex->def);
diff --git a/libraries/net/lwip/lwip-sys/arch/sys_arch.h b/libraries/net/lwip/lwip-sys/arch/sys_arch.h
index 909532eb2f..620b734b30 100644
--- a/libraries/net/lwip/lwip-sys/arch/sys_arch.h
+++ b/libraries/net/lwip/lwip-sys/arch/sys_arch.h
@@ -40,8 +40,12 @@ typedef struct {
     osMutexId    id;
     osMutexDef_t def;
 #ifdef CMSIS_OS_RTX
+#ifdef __MBED_CMSIS_RTOS_CA9
+    int32_t      data[4];
+#else
     int32_t      data[3];
 #endif
+#endif
 } sys_mutex_t;
 
 // === MAIL BOX ===