Is the result from $this->query returning the expected result object? Is the return value really "empty", or FALSE (which would be returned when either no usernames match or when mysql_result fails). Use var_dump or some other debugging functions to spot problems rather than relying on error messages being raised to tell you what's wrong.

It's difficult to help you with debugging when we just get "help, it doesn't work" style requests. Debug everything, anything possibly related to the problem to make sure things are behaving as you expect (or not behaving!).