从字符串转换日期和/或时间时转换失败 - 但不知道为什么

我有下表：

USE [junglegymSQL]
GO
ALTER TABLE [dbo].[UserSession] DROP CONSTRAINT [FK_UserSession_UserID]
GO
ALTER TABLE [dbo].[UserSession] DROP CONSTRAINT [DF__UserSessi__Creat__5CA1C101]
GO
ALTER TABLE [dbo].[UserSession] DROP CONSTRAINT [DF__UserSessi__Creat__5BAD9CC8]
GO
/****** Object:  Table [dbo].[UserSession]    Script Date: 16/09/2013 3:44:55 PM ******/
DROP TABLE [dbo].[UserSession]
GO
/****** Object:  Table [dbo].[UserSession]    Script Date: 16/09/2013 3:44:55 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[UserSession](
    [UserId] [int] NOT NULL,
    [SessionId] [varchar](500) NOT NULL,
    [Created] [datetime] NOT NULL,
    [CreatedBy] [varchar](50) NOT NULL,
    [LastModifed] [datetime] NULL,
    [LastModifiedBy] [varchar](50) NULL,
 CONSTRAINT [PK_SessionID] PRIMARY KEY CLUSTERED 
(
    [SessionId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
)
GO
SET ANSI_PADDING OFF
GO
ALTER TABLE [dbo].[UserSession] ADD  DEFAULT (user_name()) FOR [Created]
GO
ALTER TABLE [dbo].[UserSession] ADD  DEFAULT (getdate()) FOR [CreatedBy]
GO
ALTER TABLE [dbo].[UserSession]  WITH CHECK ADD  CONSTRAINT [FK_UserSession_UserID] FOREIGN KEY([UserId])
REFERENCES [dbo].[User] ([UserId])
GO
ALTER TABLE [dbo].[UserSession] CHECK CONSTRAINT [FK_UserSession_UserID]
GO

我正在尝试使用以下PHP写入：

$sessiontoken = dechex(mt_rand(0, 2147483647)) . dechex(mt_rand(0, 2147483647));
// Save our cookie 'qcore' with the users session id
setcookie("qcore", $sessiontoken);
$query = "
    INSERT INTO dbo.UserSession ( 
        UserId ,
        SessionId
    ) VALUES  (
        :userid ,
        :sessionid  
    )
";
$query_params = array( 
    ':userid' => intval($row['UserId']), 
    ':sessionid' => $sessiontoken 
); 
try 
{ 
    // Execute the query to create the user 
    $stmt = $db->prepare($query); 
    $result = $stmt->execute($query_params); 
} 
catch(PDOException $ex) 
{ 
    // Note: On a production website, you should not output $ex->getMessage(). 
    // It may provide an attacker with helpful information about your code.  
    // die("Failed to run query: " . $ex->getMessage()); 
    die("Failed to run query: " . $ex->getMessage()); 
} 

但是，每次执行此操作时，都会收到以下错误：

无法运行查询： SQLSTATE[22007]： [Microsoft][SQL Server Native 客户端 11.0][SQL Server]转换日期和/或时转换失败 来自字符串的时间。

我已经检查过，以下行返回一个整数：

':userid' => intval($row['UserId']), 

鉴于我的第二列是瓦尔查尔，好吧，我很困惑。为什么当我根本不使用日期并且传递整数值时会看到此错误？