Radio buttons are not mutually exclusive in Edit page

I have two emails for a contact. The first email is primary.While saving
a new contact I can only set one radio button for primary, but in edit
page the primary radio button can be set for both emails(not mutually
exclusive)

View
<tr>
   <% for email in @contact.emails %>
   <% fields_for "contact[email_attributes][]", email do |e| %>
   <td nowrap class="formLabel">Email </td>
   <td > <%= e.select ('email_type',%w{Business Personal},
:include_blank => false) %>
   <%= e.text_field :email,:size=>"35",:maxlength=>"80" %>
   <%= e.radio_button ('isprimary', 'true') %>Primary
   </td>
</tr>
<% end %>
<% end %>
</table>