while ((node = *prev)) {
message = node->message;
in_reply_to = _notmuch_message_get_in_reply_to (message);
- if (in_reply_to &&
+ if (in_reply_to && strlen (in_reply_to) &&
g_hash_table_lookup_extended (thread->message_hash,
in_reply_to, NULL,
(void **) &parent))