title: NoMethodError in Posts#new
body: <p>I'm doing the <a href="http://guides.rubyonrails.org/getting_started.html" rel="nofollow">Getting started with Rails tutorial</a> and when I run the local server from shell I get this:</p>
  
  <pre><code>`NoMethodError in Posts#new` `/_form.html.erb where line #1 raised:
  `undefined method `model_name' for NilClass:Class
  </code></pre>
  
  <p>That is the extracted source (around line #1):</p>
  
  <pre><code>1: &lt;%= form_for @post do |f| %&gt;
  2:  &lt;% if @post.errors.any? %&gt;
  3:  &lt;div id="errorExplanation"&gt;
  4:      &lt;h2&gt;&lt;%= pluralize(@post.errors.count, "error") %&gt; prohibited
  </code></pre>
  
  <p>I just started on Ruby on Rails and I can't figure out what is happening. What am I doing wrong?</p>