diff --git a/plugins/sudoers/timestamp.c b/plugins/sudoers/timestamp.c
index ace568e5a..dfc9bdfc5 100644
--- a/plugins/sudoers/timestamp.c
+++ b/plugins/sudoers/timestamp.c
@@ -643,8 +643,8 @@ timestamp_lock(void *vcookie, struct passwd *pw)
 	if (entry.size == sizeof(struct timestamp_entry_v1)) {
 	    /* Old sudo record, convert it to TS_LOCKEXCL. */
 	    entry.type = TS_LOCKEXCL;
-	    memset((char *)&entry + offsetof(struct timestamp_entry, type), 0,
-		nread - offsetof(struct timestamp_entry, type));
+	    memset((char *)&entry + offsetof(struct timestamp_entry, flags), 0,
+		nread - offsetof(struct timestamp_entry, flags));
 	    if (ts_write(cookie->fd, cookie->fname, &entry, 0) == -1)
 		debug_return_bool(false);
 	} else {