[Rails] NilClass in Partial

I appreciate your help in advance; I feel like the issue is staring me
in the face!

I have two models: event and topic

/models/event.rb:
class Event < ActiveRecord::Base
..
has_many :topics
..

/models/topic.rb:
class Topic < ActiveRecord::Base
belongs_to :event
end

I'm successfully showing the dependent topics with the event:
/views/events/show.html.erb:

..
<table>
<%@event.topics.each do |topic| %>
<tr>
<td> <%= topic.Name %> </td>
<td> <%= topic.Description %></td>
</tr>
<% end %>
</table>
..

When I attempt to edit the record that I can successfully show, I get:

Showing /views/events/_topic.html.erb where line #2 raised:

undefined method `new_record?' for nil:NilClass
Extracted source (around line #2):

1: <div class="topic">
2: <% new_or_existing = topic.new_record? ? 'new': 'existing' %>
3: <% prefix = "event[#{new_or_existing}_topic_attributes][]"%>
4:
5: <% fields_for prefix, topic do |topic_form| -%>

The partial is rendered in edit.html.erb:

<h1>Editing tag</h1>

<%= render 'form' %>
<div id="topicContentArea">
<%= render 'topic', :collection => @event.topics %>
</div>
<%= link_to 'Show', @event %> |
<%= link_to 'Back', events_path %>

Ideas are appreciated!
Thanks,
Karl

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