Why grails throwing null pointer exception while accessing hasMany relationship first time?

I have a strange problem.
I have two domain classes User and Post with fields:

class User {
  String name
  static hasMany = [posts: Post]
  static constraints = { }
}

and

class Post {
  String content
  long date = System.getTimeInMillis()
  static constraints = { }

  static belongsTo = [user: User]
  static mapping = {
    version: 'false'
  }
}

and controller code is:

class UserController {
  def addUser = {
    def user
    if (User.count() == 0) {
      user = new User()
      user.name = "Manish Zedwal"
      user.save(flush: true)
    } else {
      user = User.get(1)
    }
    println "Posts count: " + user.posts.size()
    render "post count: " + user.posts.size()
 }
}

For the first time while accessing url http://localhost:8080/test/user/addUser, it throws null pointer exception, but after this works fine.
This is the exception I am getting

2011-08-04 15:41:25,847 [http-8080-1] ERROR errors.GrailsExceptionResolver  - Exception occurred when processing request: [GET] /test/user/addUser
Stacktrace follows:
java.lang.NullPointerException: Cannot invoke method size() on null object
        at test.UserController$_closure2.doCall(UserController.groovy:18)
        at test.UserController$_closure2.doCall(UserController.groovy)
        at java.lang.Thread.run(Thread.java:636)

and for second time, it prints and renders fine like charm

Posts count: 0

In user domain class, coz of hasMany relationship for posts, posts is a list of Post objects then there shouldn't be null pointer exception on getting the size of empty list, rather it should be zero.

Any help appreciated