必须声明标量变量

string mystr, mysql;
            OleDbConnection myconn = new OleDbConnection();
            OleDbCommand mycmd = new OleDbCommand();
            mystr = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=H:\\C#2010\\school.mdb";
            myconn.ConnectionString = mystr;
            myconn.Open();
            mysql = "select avg(分数) from score where 学号=@no";
             = mysql;
            mycmd.Connection = myconn;
            mycmd.Parameters.Add("@no", OleDbType.VarChar, 5).Value = textBox1.Text;
            textBox2.Text = mycmd.ExecuteScalar().ToString();
            myconn.Close();
程序执行时报错“必须申明标量变量@no”，请高手指导