将数据类型nvarchar转换为int时出错 - 执行存储过程时

我有一个执行创建，编辑，删除和读取操作的存储过程 .

ALTER PROCEDURE [dbo].[CURDOpSP]    
@operation varchar(50), 
@regid int = 0,
@fname varchar(50) = null,
@lname varchar(50) = null,
@phone varchar(50) = null,
@email varchar(50) = null

AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

-- Insert statements for procedure here
if(@operation = 'Insert')
BEGIN
    insert into Register(fname,lname,phone,email,status)values(@fname,@lname,@phone,@email,0);
END

else if(@operation = 'Select')
BEGIN
    select * from Register where status = 0;
END

else if(@operation = 'Edit')
BEGIN
    update Register set fname= @fname, lname = @lname, phone = @phone, email = @email where RegisterId = @regid;

END

else if(@operation = 'Delete')
BEGIN
    Update Register set status = 1 where RegisterId = @regid;
END
END

我在Controller上调用了这个程序 .

// GET: /Employee/Create
public ActionResult Create()
{
    return View();
}

// POST: /Employee/Create
[HttpPost]
public ActionResult Create(Register register)
{           
    if (ModelState.IsValid)
    {
        db.Database.ExecuteSqlCommand("dbo.CURDOpSP @p0, @p1, @p2, @p3, @p4", "Insert", register.fname, register.lname, register.phone, register.email);
        db.SaveChanges();
        return RedirectToAction("Index");
    }
    return View(register);   
}   

// POST: /Employee/Delete/5        
public ActionResult Delete(int id)
{
    Register register = db.Registers.Find(id);
    if (register == null)
    {
        return HttpNotFound();
    }           
    db.Database.ExecuteSqlCommand("dbo.CURDOpSP @p0, @p1", "Delete",register.RegisterId); 

    db.SaveChanges();
    return RedirectToAction("Index");
}

当我尝试创建新记录时，我收到此错误：

将数据类型nvarchar转换为int时出错 . 源错误：第40行：if（ModelState.IsValid）第41行：{第42行：db.Database.ExecuteSqlCommand（“dbo.CURDOpSP @ p0，@ p1，@ p2，@ p3，@ p4”，“插入”，注册.fname，register.lname，register.phone，register.email）;第43行：db.SaveChanges（）;第44行：返回RedirectToAction（“Index”）;

其他三项业务运作良好 .

如果我将参数@regid放在存储过程中，在参数声明的末尾，如：

ALTER PROCEDURE [dbo].[CURDOpSP]    
@operation varchar(50),     
@fname varchar(50) = null,
@lname varchar(50) = null,
@phone varchar(50) = null,
@email varchar(50) = null,
@regid int = 0

那么创作就没事了 . 但现在删除功能将无法正常工作 .