strrpos() expects parameter 1 to be string, array given in functions_categories.php

[kashyyyk]

I just upgraded to 1.3.9h, my cache file suddenly filled up myDebug logs.
What it says is

[11-Feb-2011 05:29:24] PHP Warning: strrpos() expects parameter 1 to be string, array given in /EDITED/includes/functions/functions_categories.php on line 484

Here are lines 477-514

// Get all products_id in a Category and its SubCategories
// use as:
// $my_products_id_list = array();
// $my_products_id_list = zen_get_categories_products_list($categories_id)
  function zen_get_categories_products_list($categories_id, $include_deactivated = false, $include_child = true, $parent_category = '0', $display_limit = '') {
    global $db;
    global $categories_products_id_list;
 484   $childCatID = str_replace('_', '', substr($categories_id, strrpos($categories_id, '_')));

    $current_cPath = ($parent_category != '0' ? $parent_category . '_' : '') . $categories_id;

    $sql = "select p.products_id
            from " . TABLE_PRODUCTS . " p, " . TABLE_PRODUCTS_TO_CATEGORIES . " p2c
            where p.products_id = p2c.products_id
            and p2c.categories_id = '" . (int)$childCatID . "'" .
            ($include_deactivated ? " and p.products_status = 1" : "") .
            $display_limit;

    $products = $db->Execute($sql);
    while (!$products->EOF) {
      $categories_products_id_list[$products->fields['products_id']] = $current_cPath;
      $products->MoveNext();
    }

    if ($include_child) {
      $sql = "select categories_id from " . TABLE_CATEGORIES . "
              where parent_id = '" . (int)$childCatID . "'";

      $childs = $db->Execute($sql);
      if ($childs->RecordCount() > 0 ) {
        while (!$childs->EOF) {
          zen_get_categories_products_list($childs->fields['categories_id'], $include_deactivated, $include_child, $current_cPath, $display_limit);
          $childs->MoveNext();
        }
      }
    }
    return $categories_products_id_list;
  }

Any idea why this error is occuring?

I upgraded from 1.3.8, by doing a new install with a new database, I then moved all my stuff into the new database, deleted my old files and moved the new files into their place and I kept the new data base. The only thing I've added afterwards is the Facebook open graph add on.

Side note, I just put my site in down for Maintenance and I'm not getting any error logs
Dan