NoMethodError in Shelf#show

Showing /app/app/views/shelf/_textlist.html.erb where line #5 raised:

undefined method `title' for nil:NilClass
Extracted source (around line #5):
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<% entries.each { |entry| %>
<li>
<%= link_to entry.book.title, :isbn => entry.book.isbn , :action => 'edit' %>
<%= "(#{entry.book.authors} / #{entry.book.publisher})" %>
</li>
<% } %>

Trace of template inclusion: app/views/shelf/show.html.erb

Rails.root: /app

Application Trace | Framework Trace | Full Trace

Request

Parameters:

{"shelfname"=>"LILY"}

Response

Headers:

None