title: validation error - undefined method `zero?'
body: <p>Each time I call validation method on any model, I get the following error:</p>
  
  <pre><code>NoMethodError: undefined method `zero?' for nil:NilClass
  </code></pre>
  
  <p>It appears that if I remove function sum from my customized Array class the error will go away:</p>
  
  <pre><code>class Array
    def sum
      inject( nil ) { |sum,x| sum ? sum+x : x }
    end
  ..
  </code></pre>
  
  <p>Why is this happening?
  How can I fix this and still preserve the sum function? </p>