problem executing query

[eluser]Unknown[/eluser]
In a model I am writing I am saving my sql query to a variable called $sql and then doing the following

$query = $this->db->query($sql);

However this doesn't seem to be working as when I do a var_dump on the $query varaible it is empty. I am checking the sql before I run it and it does return results. Is there anything I'm doing wrong?

Thanks for any help