THREAD_DEBUG("checking message = %s in_reply_to=%s\n",
notmuch_message_get_message_id (message), in_reply_to);
- if (in_reply_to && strlen (in_reply_to) &&
+ if (in_reply_to && (! EMPTY_STRING(in_reply_to)) &&
g_hash_table_lookup_extended (thread->message_hash,
in_reply_to, NULL,
(void **) &parent)) {