diff --git a/bsp/stm32/libraries/HAL_Drivers/drivers/drv_usart.c b/bsp/stm32/libraries/HAL_Drivers/drivers/drv_usart.c
index 5c41bf2841..ca344d9ed3 100644
--- a/bsp/stm32/libraries/HAL_Drivers/drivers/drv_usart.c
+++ b/bsp/stm32/libraries/HAL_Drivers/drivers/drv_usart.c
@@ -357,7 +357,7 @@ static int stm32_putc(struct rt_serial_device *serial, char c)
 #else
     uart->handle.Instance->DR = c;
 #endif
-    while (__HAL_UART_GET_FLAG(&(uart->handle), UART_FLAG_TC) == RESET && block_timeout--);
+    while (__HAL_UART_GET_FLAG(&(uart->handle), UART_FLAG_TC) == RESET && --block_timeout);
     return (block_timeout != 0) ? 1 : -1;
 }