Using "taxonomy/autocomplete" for "#autocomplete_path' returns AJAX error. Problem with SQL statement.

I've come across this same problem on two different machines with two different sites, but I haven't seen it reported anywhere else, so maybe I'm just doing something wrong.

In a form, I want to use a textfield with an autocomplete path that points to a taxonomy vocabulary. I use this syntax:

  $form['location'] = array(
    '#title' => t('Location'),
    '#type' => 'textfield',
    '#autocomplete_path' => 'taxonomy/autocomplete/4',
    '#description' => t('Enter the location. Separate terms with commas. (eg. Los Angeles, California)')
  );

When I type anything in the text field, I get "An AJAX HTTP error occurred. HTTP Result Code: 500".

From the log message:
PDOException: SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')) AND (t.name LIKE '%at%' ESCAPE '\\') LIMIT 10 OFFSET 0' at line 1: SELECT t.tid AS tid, t.name AS name FROM {taxonomy_term_data} t WHERE (t.vid IN ()) AND (t.name LIKE :db_condition_placeholder_0 ESCAPE '\\') LIMIT 10 OFFSET 0; Array ( [:db_condition_placeholder_0] => %at% ) in taxonomy_autocomplete() (line 141 of C:\xampp\htdocs\livemusic\modules\taxonomy\taxonomy.pages.inc)

The problem seems to be with the $field_name parameter in the taxonomy_autocomplete function in taxonomy.pages.inc. In the SQL statement in the error message, no vid is set.

I managed to get the functionality I wanted by building my own autocomplete function.

I added the following to hook_menu

  $items['example/autocomplete/%'] = array(
    'title' => 'Example taxonomy autocomplete',
    'page callback' => '_example_autocomplete',
    'page arguments' => array(2),
    'access arguments' => array('use autocomplete'),
    'type' => MENU_CALLBACK
  );

And then I created a function called _example_autocomplete and copied the taxonomy_autocomplete function from taxonomy.pages.inc. I then made a couple small changes. For my case, I just need to specify one vid, so I renamed the $field_name parameter to $vid. I got rid of the "$field = field_info_field($field_name);" line, plus all of the code used to build the $vids array. Then I changed line 138 from:
->condition('t.vid', $vids)
to
->condition('t.vid', $vid, '=')
where $vid is passed as a url parameter.

I can then call this autocomplete function for a textfield type using "'#autocomplete_path' => 'example/autocomplete/4',"