Error while updating database

Question

why am i getting this error

 

The parameterized query '(STUD_NAME nvarchar(4000),STUD_ID nvarchar(4000),ca_1nvarchar' expects the parameter 'STUD_NAME', which was not supplied.

 

Here is my code

con.Open();
try
{
CalculateTotal();
for (int i = 0; i < dgAssessment.Rows.Count; i++)
{
string cb = ("update ASSESSMENTS set STUD_NAME=@STUD_NAME ,STUD_ID=@STUD_ID,CA_1=@CA_1,CA_2=@CA_2,CA_3=@CA_3,CA_4=@CA_4,EXAM=@EXAM,TOTAL=@TOTAL,GRADE=@GRADE,SUBJ_POSITION=@SUBJ_POSITION,REMARKS=@REMARKS,NO_OF_LESSONS=@NO_OF_LESSONS,ATTENDANCE=@ATTENDANCE where STUD_ID= '" + dgAssessment.Rows[i].Cells["StudentRef"].Value + "'");
cmd = new SqlCommand(cb);
cmd.Connection = con;
cmd.Parameters.AddWithValue("STUD_NAME", dgAssessment.Rows[i].Cells["Student_Name"].Value);
cmd.Parameters.AddWithValue("STUD_ID", dgAssessment.Rows[i].Cells["StudentRef"].Value);
cmd.Parameters.AddWithValue("CA_1", dgAssessment.Rows[i].Cells["ca1"].Value);
cmd.Parameters.AddWithValue("CA_2", dgAssessment.Rows[i].Cells["CA2"].Value);
cmd.Parameters.AddWithValue("CA_3", dgAssessment.Rows[i].Cells["CA3"].Value);
cmd.Parameters.AddWithValue("CA_4", dgAssessment.Rows[i].Cells["CA4"].Value);
cmd.Parameters.AddWithValue("EXAM", dgAssessment.Rows[i].Cells["EXAM"].Value);
cmd.Parameters.AddWithValue("TOTAL", dgAssessment.Rows[i].Cells["TOTAL"].Value);
cmd.Parameters.AddWithValue("GRADE", dgAssessment.Rows[i].Cells["GRADE"].Value);
cmd.Parameters.AddWithValue("SUBJ_POSITION", dgAssessment.Rows[i].Cells["Position"].Value);
cmd.Parameters.AddWithValue("NO_OF_LESSONS", dgAssessment.Rows[i].Cells["NoofLessons"].Value);
cmd.Parameters.AddWithValue("ATTENDANCE", dgAssessment.Rows[i].Cells["Attendance"].Value);
cmd.Parameters.AddWithValue("REMARKS", dgAssessment.Rows[i].Cells["Remark"].Value);
cmd.ExecuteNonQuery();
}//end for
MessageBox.Show("Record Updated Successfully");
}
catch (Exception ex)
{
MessageBox.Show(ex.Message, " ERROR MESSAGE", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
finally
{
con.Close();
}//end finally