Форум пользователей MySQL

Пытаюсь выполнить SQL-запросы:

ALTER TABLE `phpbb_topics` ADD FULLTEXT (`topic_title`);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_stopwords',1);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_ignore_forums_ids','');
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_sort_type','relev');
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_max_topics',5);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_topics_posting',1);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_topics_viewtopic',1);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_topics_min_amount_words_viewtopic', 0);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_topics_min_amount_words_posting', 0);
INSERT INTO phpbb_config (config_name, config_value) VALUES ('similar_topics_min_relevance', 0);

но получаю ошибку вида:


Общая ошибка
SQL ERROR [ mysql4 ]

Can't find FULLTEXT index matching the column list [1191]

SQL

SELECT f.forum_id, f.forum_name, t.*, u.user_id, u.username, u.user_colour, MATCH (t.topic_title) AGAINST ('нова тема' ) AS relevance FROM (phpbb_formtopics t) LEFT JOIN phpbb_formusers u ON (u.user_id = t.topic_poster) LEFT JOIN phpbb_formforums f ON (f.forum_id = t.forum_id) WHERE MATCH (t.topic_title) AGAINST ('нова тема' ) >= 1 AND t.topic_status <> 2 AND t.topic_id <> 24 GROUP BY t.topic_id ORDER BY t.topic_last_post_time LIMIT 2

BACKTRACE

FILE: includes/db/mysql.php
LINE: 174
CALL: dbal_mysql->sql_error()

FILE: includes/db/mysql.php
LINE: 221
CALL: dbal_mysql->sql_query()

FILE: includes/db/dbal.php
LINE: 170
CALL: dbal_mysql->_sql_query_limit()

FILE: includes/similar_topics.php
LINE: 132
CALL: dbal_mysql->sql_query_limit()

FILE: viewtopic.php
LINE: 1599
CALL: include('includes/similar_topics.php')


Что за ошибка, как устранить?