Return number if it already ends with zero

The specification does not mention this fact
https://www.pangaliit.ee/settlements-and-standards/reference
-number-of-the-invoice

app/models/billing/reference_no/base.rb

@@ -26,8 +26,8 @@ module Billing
         next_number = number
 
         loop do
-          next_number = next_number.next
           return next_number if next_number.to_s.end_with?('0')
+          next_number = next_number.next
         end
       end
 

test/models/billing/reference_no/base_test.rb

@@ -15,6 +15,7 @@ class ReferenceNoBaseTest < ActiveSupport::TestCase
   def test_generates_check_digit_for_a_given_base
     assert_equal 3, Billing::ReferenceNo::Base.new('1').check_digit
     assert_equal 7, Billing::ReferenceNo::Base.new('1234567891234567891').check_digit
+    assert_equal 0, Billing::ReferenceNo::Base.new('773423').check_digit
   end
 
   def test_returns_string_representation