NoMethodError in Book

I got a following error.

NoMethodError in Book#new

Showing /home/amrit/boook/app/views/book/_form.html.erb where line #1 raised:

undefined method `model_name' for NilClass:Class

The content of _form.html.erb file are:

<%= form_for(@post) do |f| %>   <% if @post.errors.any? %>     <div id="error_explanation">       <h2><%= pluralize(@post.errors.count, "error") %> prohibited this post from being saved:</h2>

      <ul>       <% @post.errors.full_messages.each do |msg| %>         <li><%= msg %></li>       <% end %>       </ul>     </div>   <% end %>

  <div class="field">     <%= f.label :name %><br />     <%= f.text_field :name %>   </div>   <div class="field">     <%= f.label :title %><br />     <%= f.text_field :title %>   </div>   <div class="field">     <%= f.label :content %><br />     <%= f.text_area :content %>   </div>   <div class="actions">     <%= f.submit %>   </div> <% end %>

Any clue? Thanks