diff --git a/targets/TARGET_STM/stm_hal_tick_32b.c b/targets/TARGET_STM/stm_hal_tick_32b.c
index bb2eec16cf..733ceabb26 100644
--- a/targets/TARGET_STM/stm_hal_tick_32b.c
+++ b/targets/TARGET_STM/stm_hal_tick_32b.c
@@ -33,7 +33,7 @@
 #define DEBUG_TICK 0 // Set to 1 to toggle a pin (see below which pin) at each tick
 
 TIM_HandleTypeDef TimMasterHandle;
-uint32_t PreviousVal = 0;
+volatile uint32_t PreviousVal = 0;
 
 void us_ticker_irq_handler(void);