Fatal error: Call to a member function SetFetchMode() on a non-object error

Question

I was using ADODB to fetch data. I get error like this:

Fatal error: Call to a member function SetFetchMode() on a non-object

and this is my code:

function createAdodbConnection($fetchMode = ADODB_FETCH_ASSOC) {
        $self = DbManager::_instance();
        $conn = &ADONewConnection($self->getDsn());
        $conn->SetFetchMode($fetchMode);
        return $conn;
    }