从数据类型 nvarchar 转换为 int 时出错。

数据库结构
bigclass(){ id,int; zname,nvarchar(50)}
midclass(){ id,int; mname,nvarchar(50),bigclass,int}
存储过程ALTER PROCEDURE insertmid@bigclass int,
@mname nvarchar(50)
AS
begininsert into midclass(mname,bigclass)
values(@bigclass,@mname)
end
DAL操作
 //增加二级分类
        public bool insertmid(string mname, string bigclass)
        {
            bool flag = false;
            string flname = "insertmid";
            SqlParameter[] pars = new SqlParameter[]{
                new SqlParameter("@mname",mname),
                new SqlParameter("@bigclass",bigclass)
            };
            int res = sqlhelper.ExecuteNonQueryt(flname, pars, CommandType.StoredProcedure);
            if (res > 0)
            {
                flag = true;
            }
            return flag;        }
表单操作namespace JK.admin
{
    public partial class adminmidclass : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                //绑定大类
                Repeater1.DataSource = new midclassmanager().selectmid();
                Repeater1.DataBind();                 //绑定大类列表
                DropDownList1.DataSource = new bigclassmanager().selectbc();
                DropDownList1.DataTextField = "zname";
                DropDownList1.DataValueField = "id";
                DropDownList1.DataBind();
            }
            
        }        protected void Button1_Click(object sender, EventArgs e)
        {
            string mname = TextBox1.Text.Trim();
            string bigclass =DropDownList1.SelectedValue;
            bool b = new midclassmanager().insertmid(bigclass, mname);
           if (b)
            {
                TextBox1.Text = "";
                Repeater1.DataSource = new midclassmanager().selectmid();
                Repeater1.DataBind();
            }
                else
       {
           Page.ClientScript.RegisterStartupScript(Page.GetType(), "message", "<script language='javascript' defer>alert('类别添加失败，请联系管理员！');</script>");
       }
        }
        protected void btnbtn_Click(object sender, EventArgs e)
        {
            LinkButton lb = (LinkButton)sender;
            string id = lb.CommandArgument;
            bool b = new midclassmanager().deletemid(id);
            if (b)
            {
                Repeater1.DataSource = new midclassmanager().selectmid();
                Repeater1.DataBind();
            }
            else
            {
                Page.ClientScript.RegisterStartupScript(Page.GetType(), "message", "<script language='javascript' defer>alert('类别删除失败，请联系管理员！');</script>");
            }
        }
    }SQLherlper
    /// <summary>
        /// 执行带参数的SQL（增删改）的语句
        /// </summary>
        /// <param name="cmdtext"></param>
        /// <param name="pars"></param>
        /// <param name="ct"></param>
        /// <returns></returns>
        public int ExecuteNonQueryt(string cmdtext, SqlParameter[] pars, CommandType ct)
        {
            int res;
            using (cmd = new SqlCommand(cmdtext, getconn()))
            {
                cmd.CommandType = ct;
                cmd.Parameters.AddRange(pars);
                res = cmd.ExecuteNonQuery();
            }
            return res;
        }