# File lib/sass/script/functions.rb, line 552
      def assert_unit(number, unit, name = nil)
        assert_type number, :Number, name
        return if number.is_unit?(unit)
        expectation = unit ? "have a unit of #{unit}" : "be unitless"
        if name
          raise ArgumentError.new("Expected $#{name} to #{expectation} but got #{number}")
        else
          raise ArgumentError.new("Expected #{number} to #{expectation}")
        end
      end