diff --git a/src/timesync/timesyncd-manager.c b/src/timesync/timesyncd-manager.c
index d7f511ee22..d7060aaa45 100644
--- a/src/timesync/timesyncd-manager.c
+++ b/src/timesync/timesyncd-manager.c
@@ -471,6 +471,8 @@ static int manager_receive_response(sd_event_source *source, int fd, uint32_t re
 
                 switch (cmsg->cmsg_type) {
                 case SCM_TIMESTAMPNS:
+                        assert(cmsg->cmsg_len == CMSG_LEN(sizeof(struct timespec)));
+
                         recv_time = (struct timespec *) CMSG_DATA(cmsg);
                         break;
                 }