Why do I get System exit message once a week or so

On an average I get one or two error messages in a week. The complete error message is given below.

SystemExit: exit: SELECT * FROM cities WHERE (
cities.id
= ‘18’) LIMIT 1
On line #15 of app/views/postevent/venue

.rhtml

12:
13:         <tr>
14:            <th>State :</th>
15:            <td><%= City.get_state_name(@draft.temp_city_id) %> </td>
16:         </tr>
17:         <tr>
18:            <th>City :</th>