db_set_active acting strangely

In settings I have:

$db_url['default'] = 'mysql://aaa';
$db_url['depletion'] = 'mysql://bbb';

In my custom module I have:

db_set_active('depletion');
.
.
.
db_set_active('default');

Even with nothing but commented out lines between them, I get the following error:

user error: Table 'local_aspo.blocks' doesn't exist
query: SELECT * FROM blocks WHERE status = 1 AND region = 0 ORDER BY weight, module in /.[snip]./database.mysql.inc on line 66.

I've tried removing the word 'default' from the 'default' db_set_active which makes no change.

BUT... when I change it to:
db_set_active('anything-else-but-default-or-depletion');

... it bizarrely works!

I'm using a two-day old 4.6RC. db_set_active is:

function db_set_active($name = 'default') {
  global $db_url, $db_type, $active_db;
  static $db_conns;

  if (!isset($db_conns[$name])) {
    // Initiate a new connection, using the named DB URL specified.
    if (is_array($db_url)) {
      $connect_url = array_key_exists($name, $db_url) ? $db_url[$name] : $db_url['default'];
    }
    else {
      $connect_url = $db_url;
    }

    $db_type = substr($connect_url, 0, strpos($connect_url, '://'));
    $handler = "includes/database.$db_type.inc";

    if (is_file($handler)) {
      include_once($handler);
    }
    else {
      die('Unsupported database type');
    }

    $db_conns[$name] = db_connect($connect_url);

  }
  // Set the active connection.
  $active_db = $db_conns[$name];
}

and in trying to debug, I did a print_r($db_conns) and got:

Array
(
[default] => Resource id #6
)

Array
(
[default] => Resource id #6
[depletion] => Resource id #6
)

... which may be the problem somehow?

I'm still trying to get to grips with Drupal's ins-and-outs, so maybe someone more familiar can help me here. Its the first time I've tried a connection to an external database.

Thanks in advance for any pointers,

Jake