diff --git a/rtos/rtx5/mbed_rtx_conf.h b/rtos/rtx5/mbed_rtx_conf.h
index e4f2f807a1..c41a8df5e0 100644
--- a/rtos/rtx5/mbed_rtx_conf.h
+++ b/rtos/rtx5/mbed_rtx_conf.h
@@ -32,7 +32,9 @@
 #define OS_STACK_SIZE               MBED_CONF_APP_THREAD_STACK_SIZE
 
 #define OS_TIMER_THREAD_STACK_SIZE 768
-#define OS_IDLE_THREAD_STACK_SIZE  512
+#ifndef OS_IDLE_THREAD_STACK_SIZE
+#define OS_IDLE_THREAD_STACK_SIZE  256
+#endif
 
 #define OS_DYNAMIC_MEM_SIZE         0