diff --git a/drivers/rtc/rtc-at91sam9.c b/drivers/rtc/rtc-at91sam9.c
index 86c61f1..78a018b 100644
--- a/drivers/rtc/rtc-at91sam9.c
+++ b/drivers/rtc/rtc-at91sam9.c
@@ -161,7 +161,7 @@
 	if (offset == 0)
 		return -EILSEQ;
 
-	memset(alrm, 0, sizeof(alrm));
+	memset(alrm, 0, sizeof(*alrm));
 	if (alarm != ALARM_DISABLED && offset != 0) {
 		rtc_time_to_tm(offset + alarm, tm);