diff --git a/arch/arm/src/stm32/stm32_serial.c b/arch/arm/src/stm32/stm32_serial.c
index 18e65f1d9a..327e515b5f 100644
--- a/arch/arm/src/stm32/stm32_serial.c
+++ b/arch/arm/src/stm32/stm32_serial.c
@@ -981,6 +981,8 @@ static void up_setspeed(struct uart_dev_s *dev)
      }
    else
      {
+       DEBUGASSERT(usartdiv8 >= 8);
+
        /* Perform mysterious operations on bits 0-3 */
 
        brr  = ((usartdiv8 & 0xfff0) | ((usartdiv8 & 0x000f) >> 1));