title: Undefined method '+' for Rspec. What does it mean?
body: <p>I'm hitting a wall because I'm terribly confused about what this rspec error message means. I am testing to see whether an art piece has a cost. Here are snippets of my rspec:</p>
  
  <pre><code>let(:valid_art_piece){ { date_of_creation: DateTime.parse('2012-3-13'), 
  placement_date_of_sale: DateTime.parse('2014-8-13'), cost: 250, medium: 'sculpture', 
  availability: true } }
  
  it 'requires a cost' do
    art_piece = ArtPiece.new(valid_art_piece.merge(cost: ''))
    expect(art_piece).to_not be_valid
    expect(art_piece.errors[:cost].to include "can't be blank")
  end
  </code></pre>
  
  <p>error message:</p>
  
  <pre><code>1) ArtPiece requires a cost
   Failure/Error: expect(art_piece.errors[:cost].to include "can't be blank")
   NoMethodError:
     undefined method `+' for #&lt;RSpec::Matchers::BuiltIn::Include:0x000001050f4cd0&gt;
   # ./spec/models/artist_piece_spec.rb:30:in `block (2 levels) in &lt;top (required)&gt;'
  </code></pre>
  
  <p>As far as I'm concerned this shouldn't have failed and I don't know why its failing. My schema.rb has the field as null false and my model validates for it with the numericality: true option. </p>
  
  <pre><code>class ArtPiece &lt; ActiveRecord::Base
    validates_presence_of :date_of_creation
    validates_presence_of :placement_date_of_sale
    validates :cost, numericality: true
    validates_presence_of :medium
    validates_presence_of :availability
  end
  </code></pre>
  
  <p>I don't know what the problem is. Some help?</p>