x.rb

class X
  attr_accessor :i
  def initialize
    @i = 0
  end  
  def work(e)
    i += e
  end  
end  

x = X.new
x.work 5 # => undefined method `+' for nil:NilClass (NoMethodError)

Yes, this is correct.

You can also do:

def work(e)
  self.i += e
end

The wise man said: https://twitter.com/roidrage/status/599580807993434113

Ruby uses local scope by default and creates the variable i in the function afair. Needs to be self.i or @i with assignments.