Problem creating a new page in C#

Hello all,

I'm getting an error when I try to add a new page into a ContentReference.

The page type (Article) has the following attribute:

[AllowedTypes(typeof(SpecialistPage))]
[Display(Name = "Auteurs", Description = "De auteurs van het artikel", Order = 40, GroupName = SystemTabNames.Content)]
public virtual IList<ContentReference> Authors { get; set; }

I'm able to create a new article page, but when I try to add a page into the Authors ContentReference block with the following code:

var cr = ServiceLocator.Current.GetInstance<EPiServer.IContentRepository>();
var articleOverview = cr.GetChildren<ArticleOverviewPage>(EPiServer.Core.ContentReference.StartPage).FirstOrDefault();
var articlePage = ContentRepository.GetDefault<ArticlePage>(articleOverview.ContentLink);
if (authorPreference != null)
{
    foreach (var author in authorPreference)
    {
        var page = ContentRepository.Get<PageData>(Guid.Parse(author));
        articlePage.Authors.Add(page.ContentLink);
    }
}

I get an exception "System.NullReferenceException: Object reference not set to an instance of an object."

I can see that the variable page is correctly created so the page.ContentLink exist, so don't know why I'm getting this error.

Thanks in advance.