Regexp.last_match in the block of gsub should not be nil

a.rb

class A
    def initialize
        @str = 'This is a string'
    end

    def method_missing(name, *args, &block)
        if @str.respond_to?(name)
            @str.send(name, *args, &block)
        else
            super
        end
    end
end

a = A.new
a.gsub(/ is /) do |m|
    puts "last_match='#{Regexp.last_match}' m='#{m}'"
    ' is not '
end