DeafaultIfEmpty() in joing query throwing integer cast exception

See more:

C#

LINQ

entity-framework

I'm using following query to get data from data base but it is throwing exception saying "An exception of type System.InvalidOperationException occurred in EntityFramework.dll but was not handled in user code
Additional information: The cast to value type System.Int64& failed because the materialized value is null. Either the result types generic parameter or the query must use a nullable type."
********************************************************

var articles = (from art in db.Article_Template
                                join uad in db.Article_Template_Detail on art.USER_ARTICLEID equals uad.USER_TemplateID into aADetail
                                from aad in aADetail.DefaultIfEmpty() orderby art.CREATED_ON descending
                                select new
                                {
                                    art.CONTENTID,
                                    art.ARTICLE_TITLE,
                                    art.ANSWERED,
                                    art.ARTICLE_DESCRIPTION,
                                    art.CONTENT_TYPE,
                                    art.CREATED_BY,
                                    art.CREATED_ON,
                                    art.IP_ADDRESS,
                                    art.ISACTIVE,
                                    art.MODIFIED_BY,
                                    art.MODIFIED_ON,
                                    art.ReleventScore,
                                    art.TAG,
                                    art.TEMPLATEID,
                                    art.SectionID,
                                    art.USER_ARTICLEID,
                                    aad.USERS_Template_DETAILID,
                                    aad.USER_Template_IMAGE_PATH,
                                    aad.USER_Template_IMAGE_THUMNAIL

                                }).Take(1000).ToList();

********************************************************
However when I execute this query the following way(i.e without DefaultIfEmpty()) it works

var articles = (from art in db.Article_Template
                                    join uad in db.Article_Template_Detail on art.USER_ARTICLEID equals uad.USER_TemplateID
                                    orderby art.CREATED_ON descending
                                    select new
                                    {
                                        art.CONTENTID,
                                        art.ARTICLE_TITLE,
                                        art.ANSWERED,
                                        art.ARTICLE_DESCRIPTION,
                                        art.CONTENT_TYPE,
                                        art.CREATED_BY,
                                        art.CREATED_ON,
                                        art.IP_ADDRESS,
                                        art.ISACTIVE,
                                        art.MODIFIED_BY,
                                        art.MODIFIED_ON,
                                        art.ReleventScore,
                                        art.TAG,
                                        art.TEMPLATEID,
                                        art.SectionID,
                                        art.USER_ARTICLEID,
                                        uad.USERS_Template_DETAILID,
                                        uad.USER_Template_IMAGE_PATH,
                                        uad.USER_Template_IMAGE_THUMNAIL

                                    }).Take(1000).ToList()

I need to how I can fix this as my client is waiting and I'm stuck in this issue.

Thanks
Sohaib Javed