Linq to sql error＆ldquo;根级别的数据无效＆rdquo;可以为空的xml列

CREATE TABLE [dbo].[tab1]( 
 [Id] [int] NOT NULL, 
 [Name] [varchar](100) NOT NULL, 
 [Meta] [xml] NULL, 


CONSTRAINT [PK_tab1] PRIMARY KEY CLUSTERED  
( 
     [Id] ASC 
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY] 
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

     var obj1= (from obj in dbContext.tab1s
  where obj.id== 123
select obj).FirstOrDefault<Tab1>();