ASP.Net ExecuteScalar Error: Must declare the scalar variable

I have an insert statement in the asp web form that before inserting the data into the database check it for duplicate data.

this is my code that avoids inserting duplicate data: 

                string constr = ConfigurationManager.ConnectionStrings["KDUIS-v1ConnectionString"].ConnectionString;
                using (SqlConnection con = new SqlConnection(constr))
                {
                    using (SqlCommand cmd = new SqlCommand("SELECT * FROM EDU_Professor WHERE PersonId = @PersonId", con))
                    {
                        cmd.Parameters.AddWithValue("@PersonId", Convert.ToInt32(LblPersonId.Text));
                        using (SqlCommand cmd2 = new SqlCommand("SELECT * FROM EDU_SubjectStudy_Professor WHERE SubjectStudyId=@SubjectStudyId", con))
                        {
                            cmd.Parameters.AddWithValue("@SubjectStudyId",Convert.ToInt32(LblSubjectStudyId.Text));
                            con.Open();
                            string name = Convert.ToString(cmd.ExecuteScalar());
                            string name2 = Convert.ToString(cmd2.ExecuteScalar());
                            con.Close();
                            if (string.IsNullOrEmpty(name)&& string.IsNullOrEmpty(name2))
                            {
                                sqlDtSrcProfessor.Insert();
                                int ProfessorId = ProfId();
                                SqlDtSrcSSProf.Insert();
                                ScriptManager.RegisterStartupScript(this, this.GetType(), "CropImage", "alert('your data is inserted')", true);
                                TxtCode.Text = "";
                                txtday1.Text = "";
                                txtday2.Text = "";
                                TxtDescription.Text = "";
                                txtmonth1.Text = "";
                                txtmonth2.Text = "";
                                TxtProfessor.Text = "";
                                ListSubjectStudy.SelectedIndex = 0;
                            }
                            else
                            {
                                ScriptManager.RegisterStartupScript(this, this.GetType(), "CropImage", "alert('this field is duplicate')", true);
                            }
                        }
                       
                    }
                }

but when I want to run my code, I faced this error message.

and the type of SubjectStudyId is int.