I'm trying to get the basic of Treetop parsing. Here's a very simple bit of grammar so that I can say ArithmeticParser.parse('2+2').value == 4.

grammar Arithmetic
  rule additive
    first:number '+' second:number {
      def value
        first.value + second.value
      end
    }
  end

  rule number
    [1-9] [0-9]* {
      def value
        text_value.to_i
      end
    }
  end
end

Parsing 2+2 works correctly. However, parsing 2 or 22 returns nil.

What did I miss?