diff --git a/lib/stm32/common/rtc_common_l1f024.c b/lib/stm32/common/rtc_common_l1f024.c
index 2d529449..d7825a7c 100644
--- a/lib/stm32/common/rtc_common_l1f024.c
+++ b/lib/stm32/common/rtc_common_l1f024.c
@@ -107,6 +107,7 @@ void rtc_set_wakeup_time(uint16_t wkup_time, uint8_t rtc_cr_wucksel)
 	 *    down-counting.
 	 */
 	RTC_WUTR = wkup_time;
+	RTC_CR &= ~(RTC_CR_WUCLKSEL_MASK << RTC_CR_WUCLKSEL_SHIFT);
 	RTC_CR |= (rtc_cr_wucksel << RTC_CR_WUCLKSEL_SHIFT);
 	RTC_CR |= RTC_CR_WUTE;
 }