See my response to necubi. The main problem is the reverse:

  if some_value
    something with some_value
  end

where you first use nil to signal false, but later change the base case of some_value from nil to 0, nil to NullObject or nil to some other base value. Since you think of these base cases as 'nothing', you will forget to change this related conditional the first time around, because it seems it would still do the right thing.