diff --git a/drivers/tty/serial/stm32-usart.h b/drivers/tty/serial/stm32-usart.h
index 41d97492310271dbb3f2036d484b7b2ae5ee8c10..cd97ceb76e4ffe6f48af1a614532bdf42318e2de 100644
--- a/drivers/tty/serial/stm32-usart.h
+++ b/drivers/tty/serial/stm32-usart.h
@@ -31,7 +31,7 @@ struct stm32_usart_info {
 	struct stm32_usart_config cfg;
 };
 
-#define UNDEF_REG ~0
+#define UNDEF_REG 0xff
 
 /* Register offsets */
 struct stm32_usart_info stm32f4_info = {