var Jsondata=[{MembersMobile:'13612268339',MembersName:'test',MembersSex:'男',MembersAge:'23',MembersBirthday:'NULL',MembersIDCard:'NULL',MembersMarital:'NULL',MembersChildren:'NULL',MembersJob:'NULL',MembersIncome:'NULL',MembersLiveCity:'NULL',MembersHouseCharecter:'NULL',MembersLiveAddress:'NULL',MembersLiveZipCode:'NULL',MembersPostAddress:'NULL',MembersPostZipCode:'NULL',MembersJobCity:'NULL',MembersCompany:'NULL',MembersDept:'NULL',MembersPosition:'NULL',MembersEmail:'NULL',MembersQQ:'NULL',MembersMSN:'NULL',MembersEmail:'NULL',MembersMobileType:'NULL',MembersSIMType:'NULL',MembersGPRS:'NULL',MembersRank:'NULL',MembersAscription:'NULL',MembersSourceID:'NULL',MembersLoninID:'NULL',MembersPassword:'NULL',MembersFlag:'NULL',RecorderId:'NULL',RecordTime:'NULL',DelecteFlag:'NULL',BlackMenuFlag:'NULL'};


  MembersTbl[] MembersTbls = JsonConvert.DeserializeObject <MembersTbl[]>(Jsondata);

在这里出错：
could not cast or convert from system.string to sysmtem.nullable'[system.datatime]
是什么原因?