search multiple listing category terms

I have changed the listing_category form item in the ulistingblock.module to allow multiple selections as well as in the drupal listing_category taxanomy but this causes the search to throw up error messages;

# warning: Illegal offset type in isset or empty in C:\xampp\htdocs\drupal-6.10\modules\taxonomy\taxonomy.module on line 1015.
# warning: Illegal offset type in C:\xampp\htdocs\drupal-6.10\modules\taxonomy\taxonomy.module on line 1016.
# warning: Illegal offset type in C:\xampp\htdocs\drupal-6.10\modules\taxonomy\taxonomy.module on line 1019.
# warning: Illegal offset type in isset or empty in C:\xampp\htdocs\drupal-6.10\modules\taxonomy\taxonomy.module on line 842.
# user warning: Unknown column 'Array' in 'where clause' query: SELECT COUNT(DISTINCT(L.nid)) FROM ulisting_listing L INNER JOIN node N on L.nid = N.nid LEFT JOIN term_node tn0 ON N.vid = tn0.vid WHERE N.status = 1 AND N.type = 'ulisting' AND L.price >= 0 AND L.price <= 4950000 AND tn0.tid IN (Array) in C:\xampp\htdocs\drupal-6.10\sites\businessbroking.com.au\modules\ulisting\ulistingblock\ulistingblock.module on line 613.
# user warning: Unknown column 'Array' in 'where clause' query: SELECT DISTINCT(L.nid), L.price,L.bedroom,L.bathroom_full, L.address,L.city_name,N.title,L.currency,C.code FROM ulisting_listing L INNER JOIN node N on L.nid = N.nid INNER JOIN country C on L.country_id = C.country_id LEFT JOIN term_node tn0 ON N.vid = tn0.vid WHERE N.status = 1 AND N.type = 'ulisting' AND L.price >= 0 AND L.price <= 4950000 AND tn0.tid IN (Array) ORDER BY price asc LIMIT 0, 10 in C:\xampp\htdocs\drupal-6.10\sites\businessbroking.com.au\modules\ulisting\ulistingblock\ulistingblock.module on line 613.

I have tried changing other form items to allow multiple selections and it has given the same error. Is the SQL statement built to only handle single taxonomy selections?

I have also tried using the Hierarchical Select module to allow users a simpler way to choose a listing category but this is also causing the same errors when changing the form item in ulistingblock.module.

Hope you can help or suggest a different way to have multiple sets of category terms. Cheers.