Documentation problem with db_set_active

API page: http://api.drupal.org/api/drupal/includes--database.inc/function/db_set_...

I'm actually confused with db_set_active documentation.
It says:

Return value
the name of the previously active database or FALSE if non was found.

In my logic mind, I see this sentence as follows:
Return value: the name of the previously active database (IF SUCCEEDED) or FALSE if non was found (FAILED).

So I'm expecting as follows:
db_set_active('bad_database') to return FALSE, because it doesn't exists ("FALSE if non was found")
db_set_active('correct_database') to return my previous database, which is 'default' (BTW. not 'correct_database')

But in practice, db_set_active() never returns FALSE, no matter if requested database exists or not.

See the backtrace:

-> db_set_active($name = 'datagov_wiki_live') sites/all/modules/_custom/data_gov_uk_custom/data_gov_uk_custom.module:1121
  -> is_array('mysql://root:root@localhost/datagov_live') includes/database.inc:135
   >=> FALSE
 => $connect_url = 'mysql://root:root@localhost/datagov_live' includes/database.inc:146
  -> is_array(NULL) includes/database.inc:147
   >=> FALSE
 => $slave_connect_url = NULL includes/database.inc:152 
  -> strpos('mysql://root:root@localhost/datagov_live', '://') includes/database.inc:156
   >=> 5
  -> substr('mysql://root:root@localhost/datagov_live', 0, 5) includes/database.inc:156
   >=> 'mysql' 
 => $db_type = 'mysql' includes/database.inc:156
 => $handler = './includes/database.mysql.inc' includes/database.inc:157
  -> is_file('./includes/database.mysql.inc') includes/database.inc:159
   >=> TRUE
  -> db_connect($url = 'mysql://root:root@localhost/datagov_live') includes/database.inc:166
    -> parse_url('mysql://root:root@localhost/datagov_live') includes/database.mysql.inc:52
     >=> array ('scheme' => 'mysql', 'host' => 'localhost', 'user' => 'root', 'pass' => 'root', 'path' => '/datagov_live')
   => $url = array ('scheme' => 'mysql', 'host' => 'localhost', 'user' => 'root', 'pass' => 'root', 'path' => '/datagov_live') includes/database.mysql.inc:52
    -> function_exists('mysql_connect') includes/database.mysql.inc:55
     >=> TRUE
    -> urldecode('root') includes/database.mysql.inc:60
     >=> 'root'
   => $url['user'] = 'root' includes/database.mysql.inc:60
    -> urldecode('root') includes/database.mysql.inc:62
     >=> 'root'
   => $url['pass'] = 'root' includes/database.mysql.inc:62
    -> urldecode('localhost') includes/database.mysql.inc:63
     >=> 'localhost' 
   => $url['host'] = 'localhost' includes/database.mysql.inc:63
    -> urldecode('/datagov_live') includes/database.mysql.inc:64
     >=> '/datagov_live'
   => $url['path'] = '/datagov_live' includes/database.mysql.inc:64
    -> mysql_connect('localhost', 'root', 'root', TRUE, 2) includes/database.mysql.inc:77
     >=> resource(295) of type (mysql link)
   => $connection = resource(295) of type (mysql link) includes/database.mysql.inc:77
    -> substr('/datagov_live', 1) includes/database.mysql.inc:78
     >=> 'datagov_live'
    -> mysql_select_db('datagov_live') includes/database.mysql.inc:78
     >=> TRUE
    -> mysql_query('SET NAMES utf8', resource(295) of type (mysql link)) includes/database.mysql.inc:90
     >=> TRUE
   >=> resource(295) of type (mysql link)
 => $db_conns['datagov_wiki_live'] = resource(295) of type (mysql link) includes/database.inc:166
 => $previous_name = 'default' includes/database.inc:172 => $active_name = 'datagov_wiki_live' includes/database.inc:174
 => $active_db = resource(295) of type (mysql link) includes/database.inc:175
 >=> 'default'

Test code:

$ drush eval "var_dump(db_set_active('blah'));"
string(7) "default"