diff --git a/rtos/TARGET_CORTEX/mbed_rtx_conf.h b/rtos/TARGET_CORTEX/mbed_rtx_conf.h
index 91db38a617a..bd4fb4c6753 100644
--- a/rtos/TARGET_CORTEX/mbed_rtx_conf.h
+++ b/rtos/TARGET_CORTEX/mbed_rtx_conf.h
@@ -45,10 +45,18 @@
 #else
 #define EXTRA_IDLE_STACK 0
 #endif
+
+// Increase the idle thread stack size when debug is enabled
+#if defined(MBED_DEBUG)
+#define EXTRA_IDLE_STACK_DEBUG MBED_CONF_RTOS_IDLE_THREAD_STACK_SIZE_DEBUG_EXTRA
+#else
+#define EXTRA_IDLE_STACK_DEBUG 0
+#endif
+
 #ifdef MBED_CONF_APP_IDLE_THREAD_STACK_SIZE
-#define OS_IDLE_THREAD_STACK_SIZE   (MBED_CONF_APP_IDLE_THREAD_STACK_SIZE + EXTRA_IDLE_STACK)
+#define OS_IDLE_THREAD_STACK_SIZE   MBED_CONF_APP_IDLE_THREAD_STACK_SIZE
 #else
-#define OS_IDLE_THREAD_STACK_SIZE   (MBED_CONF_RTOS_IDLE_THREAD_STACK_SIZE + EXTRA_IDLE_STACK)
+#define OS_IDLE_THREAD_STACK_SIZE   (MBED_CONF_RTOS_IDLE_THREAD_STACK_SIZE + EXTRA_IDLE_STACK + EXTRA_IDLE_STACK_DEBUG)
 #endif
 
 #define OS_DYNAMIC_MEM_SIZE         0
diff --git a/rtos/mbed_lib.json b/rtos/mbed_lib.json
index 76e5321c672..3e189ee84ba 100644
--- a/rtos/mbed_lib.json
+++ b/rtos/mbed_lib.json
@@ -21,12 +21,22 @@
          "idle-thread-stack-size-tickless-extra": {
             "help": "Additional size to add to the idle thread when tickless is enabled and LPTICKER_DELAY_TICKS is used",
             "value": 256
+         },
+         "idle-thread-stack-size-debug-extra": {
+            "help": "Additional size to add to the idle thread when code compilation optimisation is disabled",
+            "value": 0
          }
     },
     "macros": ["_RTE_"],
     "target_overrides": {
         "*": {
             "target.boot-stack-size": "0x400"
+        },
+        "STM": {
+            "idle-thread-stack-size-debug-extra": 128
+        },
+        "STM32L1": {
+            "idle-thread-stack-size-debug-extra": 512
         },
 	    "MCU_NRF51": {
 	        "target.boot-stack-size": "0x800"