title: Range unspecified. Specify the :in, :within, :maximum, :minimum, or :is option
body: <p>i added a validation a my model "entrada":</p>
  
  <pre><code>class Entrada &lt; ActiveRecord::Base
    validates :titulo, presence: true,
              length: { minimun: 5 }
  
    attr_accessible :entrada, :titulo
  end
  </code></pre>
  
  <p>and so i get the following error: </p>
  
  <pre><code>Range unspecified. Specify the :in, :within, :maximum, :minimum, or :is option.
  </code></pre>