Compiler issue with non-existent NIL conditions

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FYI: final version posted to Rosetta Code.

struct PalindromicGapfuls
  include Enumerable(UInt64)

  @nn : Int32

  def initialize(@digit : Int32)
    @nn = @digit * 11                    # digit gapful divisor
  end

  def each
    (2..).each do |power|
      base    = 10_u64 ** (power >> 1)  # value of middle digit position
      this_lo = base * @digit           # starting half for this digit: 10.. to  90.. 
      next_lo = base * (@digit + 1)     # starting half for next digit: 20.. to 100..
      this_lo.step(to: next_lo - 1, by: 10) do |front_half|   # d_10; d_20; d_30; ...
        if power.odd?                   # for even number of total digits: ab..ba
          front_half.upto(front_half + 9) do |upper_half|     # d_n0 to d_n9
            palindrome_half = upper_half.to_s
            palindrome = (palindrome_half + palindrome_half.reverse).to_u64
            yield palindrome if palindrome % @nn == 0
          end
        else                           # for odd number of total digits: ab..m..ba
          palindrome_half = (front_half // 10).to_s
          palindrome = (palindrome_half + "0" + palindrome_half.reverse).to_u64
          # increase palindome middle digit from ---0--- to ---9---
          10.times do
            yield palindrome if palindrome % @nn == 0
            palindrome += base
          end
        end
      end
    end
  end
end

count, keep = 20, 20
puts "First 20 palindromic gapful numbers ending with:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }
  
count, keep = 100, 15
puts "\nLast 15 of first 100 palindromic gapful numbers ending in:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }
     
count, keep = 1_000, 10
puts "\nLast 10 of first 1000 palindromic gapful numbers ending in:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }

count, keep = 100_000, 1
puts "\n100,000th palindromic gapful number ending with:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }

count, keep = 1_000_000, 1
puts "\n1,000,000th palindromic gapful number ending with:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }

count, keep = 10_000_000, 1
puts "\n10,000,000th palindromic gapful number ending with:"
1.upto(9) { |digit| puts "#{digit} : #{PalindromicGapfuls.new(digit).first(count).last(keep)}" }