[sequelize] validation gets messed up when updating an model instance

app.coffee

# Modules
sequelize = require('sequelize')

# Connect to database
Sequelize = new sequelize('sequelize', 'sequelize', 'sequelize-test',
  host: 'localhost'
  port: '3306'
  dialect: 'mysql'

  syncOnAssociation: false
  maxConcurrentQueries: 100
)

# User model
modelUser = Sequelize.define('User',
    id:
      type: sequelize.INTEGER
      allowNull: false
      primaryKey: true
      unique: true
      autoIncrement: true

    username:
      type: sequelize.STRING(20)
      allowNull: false
      validate:
        isAlphanumeric:
          args: true
          msg: 'Username can only contain alphanumeric values'
        len:
          args: [3, 20]
          msg: 'Username must be between 3 and 20 characters in length'
)

# Force cre tables
Sequelize.sync({force: true}).success(->
  # Create a new model instance and save it into the database
  model = modelUser.create(
    username: 'admin'
  ).success(->
    # Try to find the user in the database
    modelUser.find(
      where: username: 'admin'
    ).success((user) ->
      if !user then throw new Error('Could not find user')

      # User was found, modify its username and try to save
      user.username = 'wootroot'
      user.save().success(->
        console.log('Everything went fine!')
      ).error((err) ->
        # It will end here by saying that the username hasn't got the correct length
        console.log('Something\'s wrong #2: ')
        console.log(err)
      )
    )
  )
).error((err) ->
  console.log('Something\'s wrong #1: ' + err.toString())
)