What am i doing wrong here ?

error.txt

ReferenceError: session is not defined
    at Object.<anonymous> (eval at <anonymous> (/usr/local/lib/node/.npm/coffeekup/0.2.2/package/lib/coffeekup.js:186:12))
    at anonymous (eval at <anonymous> (/usr/local/lib/node/.npm/coffeekup/0.2.2/package/lib/coffeekup.js:186:12))
    at Object.render (/usr/local/lib/node/.npm/coffeekup/0.2.2/package/lib/coffeekup.js:206:12)
    at Object.render (/usr/local/lib/node/.npm/zappa/0.1.4/package/lib/zappa.coffee:472:26)
    at Object.<anonymous> (eval at <anonymous> (/usr/local/lib/node/.npm/zappa/0.1.4/package/lib/zappa.coffee:626:12))
    at RequestHandler.anonymous [as handler] (eval at <anonymous> (/usr/local/lib/node/.npm/zappa/0.1.4/package/lib/zappa.coffee:626:12))
    at RequestHandler.execute (/usr/local/lib/node/.npm/zappa/0.1.4/package/lib/zappa.coffee:443:21)
    at Object.<anonymous> (/usr/local/lib/node/.npm/zappa/0.1.4/package/lib/zappa.coffee:250:26)
    at param (/usr/local/lib/node/.npm/connect/0.5.6/package/lib/connect/middleware/router.js:147:21)
    at pass (/usr/local/lib/node/.npm/connect/0.5.6/package/lib/connect/middleware/router.js:163:10)

tasklist.coffee

get '/': -> redirect 'index'

get '/index': ->
  render 'index'

put '/index': ->
  session.tasks ?= []
  session.tasks[session.tasks.length]=@subject
  redirect 'index'

layout ->
  head -> title 'todolist-zappa'
  body -> @content

view index: ->
  h1 'Todo-list Zappa'
  if session.tasks?
    ul ->
      for l in session.tasks
        li l
  form ->
    input type:'text', name:'subject'
    input type:'submit'