[puts 'Loren']

I just started to teach myself ruby as a first language from Chris Pines's book. There is a exercise to write a program that will translate arabic numbers to old style roman numbers.
I have completed the exercise and my code is works but only if I type in numbers that are not divisible by 5. As soon I type in a number divisible by 5 I get the following error: No implicit conversion from nil to integer (Type error)
I completely stuck.

I would be really grateful if someone could look at my code and explain what am I doing wrong.

puts 'Please type in a number that you would like to be translated'
puts 'to old style Roman number:'

def romnum leftover
var_i = 'I'
var_v = 'V'
var_x = 'X'
var_l = 'L'
var_c = 'C'
var_d = 'D'
var_m = 'M'

leftover = ''

leftover = gets.chomp

numb_m = leftover.to_i / 1000
leftover = leftover.to_i % 1000

if leftover != 0
numb_d = leftover / 500
leftover = leftover % 500

if leftover != 0
numb_c = leftover / 100
leftover = leftover % 100

if leftover != 0
numb_l = leftover / 50
leftover = leftover % 50

if leftover != 0
numb_x = leftover /10
leftover = leftover % 10

if leftover != 0
numb_v = leftover / 5
leftover = leftover % 5

if leftover != 0
numb_i = leftover /1
leftover = 0
end
end
end
end
end
end


puts var_m * numb_m + var_d * numb_d + var_c * numb_c + var_l * numb_l + var_x * numb_x + var_v * numb_v + var_i * numb_i
end

romnum 5