Find this code in the install script and remove it:
PHP Code:
$query[] = "ALTER TABLE thread DROP articleid";
$explain[] = "Removing from table <i>thread</i> the <font color='#FF3300'>articleid</font> field... Done.";
$query[] = "ALTER TABLE thread DROP INDEX articleid";
$explain[] = "Removing the <b>thread</b> table the <font color='#FF3300'>articleid</font> index... Done.";