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

I'm trying to execute a query but keep getting an error Fatal error: Call to a member function fetch_assoc() on a non-object. I have searched through forums for answers but i'm very new to php and a lot of the stuff I found is very advanced for me to get my head around. I'm working with Joomla, the connection to the database is working and I can successfully write to the table I am working with but can't select the information from it without getting this error.

                    $message = '';
                    $db = JFactory::getDBO();
                    if($db->connect_error)
                    {
                        //if error assign to message
                        $message = $db->connect_error;
                    }
                    else

                    {
                        $sql = 'SELECT * FROM jos_offer';
                        $result = $db->query($sql);
                        if($db->error)
                        {
                            $message = $db->error;
                        }
                        else
                        {
                            while($row = $result->fetch_assoc())

                            {
                                echo '
'.$row['student_id'].$row['offer'].'
'; 
                            }
                        }
                    }
                ?>