How to convert type from `Node?` to `Node` ?

node.cr

class Node
  property value : String
  property next : self | Nil

  def initialize(@value : String | Nil)
  end

  def append(node : self)
    last : self = self
    while last.next
      last = last.next
    end
    last.next = node
  end

  def values
    result = [] of String?
    current = self
    while !current.nil?
      result << current.value
      current = current.next  
    end
    result
  end
end

first : Node = Node.new("first")
second : Node = Node.new("second")
first.append(second)

puts first.values

stdout.log

Error in list/node.cr:30: instantiating 'Node#append(Node)'

first.append(second)
      ^~~~~~

in list/node.cr:12: type must be Node, not (Node | Nil)

      last = last.next
      ^~~~