Class error

How can I use methods from pear db in some functions from my class?

I have extended the class DB_mssql to my class (class index extends DB_mssql{} and I am trying to call a method from DB_mssql:

$variable = $this->getAssoc("EXECUTE procedure ? " , false , array( "code"=>1 ) );

and I am receiving an error:

DB_Error Object
(
[error_message_prefix] =>
[mode] => 1
[level] => 1024
[code] => -1
[message] => DB Error: unknown error
[userinfo] => EXECUTE procedure 1 [nativecode=-14 - Changed database context to 'DatabaseName'.]
[backtrace] => Array
(
[0] => Array
(
[file] => /usr/local/share/pear/DB.php
[line] => 966
[function] => PEAR_Error
[class] => PEAR_Error
[type] => ->
[args] => Array

And so on...

The interesting thing is that if I call that method afther objecting that function, it works perfect. The code is:

$obj=new index(); (that is my class)
$ms_sql=new DB_mssql();
$variable = $ms_sql->getAssoc("EXECUTE procedure ? " , false , array( "code"=>1 ) );

What can I do to call methods from inside of my class?

Thank you.