Why XML column is greyed out?

问题

why XML column doesn't let me put anything in it and greyed out, I can populate other columns ID A or ID B I can add values but with Xml it doesn't let me add anything, even "<element>" ?

Creating a table like this,

USE [DBNAME]
GO

DROP TABLE [dbo].[TableName]
GO

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[TableName](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [ID_A] [int] NULL,
    [ID_B] [int] NULL,
    [Xml] [xml] NULL,
 CONSTRAINT [PK_TableName] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY],
 CONSTRAINT [IX_TableName] UNIQUE NONCLUSTERED 
(
    [ID_A] ASC,
    [ID_B] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

来源：https://stackoverflow.com/questions/25032601/why-xml-column-is-greyed-out