Ruby没有将Fixnum隐式转换为String(TypeError)

Leap years. Write a program that asks for a starting year and an ending year and then puts all the leap years between them (and including them,if they are also leap years). Leap years are years divisible by 4 (like 1984 and 2004). However,years divisible by 100 are not leap years (such as 1800 and 1900) unless they are also divisible by 400 (such as 1600 and 2000,which were in fact leap years). What a mess!

当我运行我的代码时,我收到以下错误：

leap_year.rb：12：in’：没有将Fixnum隐式转换为String(TypeError)
从leap_year.rb：12：在’

这是我的代码：

#leap years

puts 'What is the starting year?'
starting_year = gets.chomp
puts 'What is the ending year?'
ending_year = gets.chomp

while starting_year <= ending_year
  if starting_year%4 == 0 && (starting_year%100 != 0 && starting_year%400 == 0)
    puts starting_year 
  end 
  starting_year+=1
end