diff --git a/src/common/tortls.c b/src/common/tortls.c
index 94c13fdfae..2ab3771f3e 100644
--- a/src/common/tortls.c
+++ b/src/common/tortls.c
@@ -627,6 +627,8 @@ tor_tls_get_peer_cert_nickname(tor_tls *tls, char *buf, size_t buflen)
   if (((int)strspn(buf, LEGAL_NICKNAME_CHARACTERS)) < lenout) {
     log_fn(LOG_WARN, "Peer certificate nickname '%s' has illegal characters.",
            buf);
+    if (strchr(buf, '.'))
+      log_fn(LOG_WARN, "  (Maybe it is not really running Tor at its advertised OR port.)");
     goto error;
   }
   X509_free(cert);