weird error

gistfile1.txt

ArgumentError: comparison of Fixnum with nil failed
	from mergesort3.rb:34:in `<'
	from mergesort3.rb:34:in `block in merge'
	from mergesort3.rb:31:in `each'
	from mergesort3.rb:31:in `merge'
	from mergesort3.rb:17:in `merge_sort'
	from mergesort3.rb:14:in `merge_sort'
	from (irb):4
	from /usr/bin/irb:12:in `<main>'



def merge_sort(m)
  #base case
  if m.length <= 1
    m
  #all other cases
  else
    #intialize variables
    left = Array.new
    right = Array.new
    middle = m.length/2

    #split first half to left and second half to right, call merge sort recursively
    left = merge_sort(m[0..middle-1])
    right = merge_sort(m[middle..-1])

    #merge left and right
    merge = merge(left, right)
    
    #return merge
    merge
  end
end

def merge(left, right)
  #initializations
  output = Array.new(left.length+right.length)
  i = 0
  j = 0

  #go through each element in the ouput array and insert a value from left or right
  output.each do |k|
    #before the counters for either left and right have ended    
    if i < left.length && j < right.length  
      if left[i] < right[j]
        k=left[i]
        i += 1
      else
        k=right[j]
        j += 1
      end
    #when counter for left has reached end, but right has not
    elsif i >= left.length && j < right.length
        k=right[j]
        j += 1
    #when counter for right has reached end, but left has not
    elsif i < left.length && j >= right.length
        k=left[i]
        i += 1    
    end
  end
  
  #return ouput
  output
end