Tell me where i am wrong

Question

An exception of type 'System.FormatException' occurred in mscorlib.dll but was not handled in user code

 

Additional information: Input string was not in a correct format. 

/DATA ACCESS LAYER

public DataSet StdCourseBindDAL(int Uid)
{
SqlConnection con = new SqlConnection(constring);
con.Open();
SqlCommand cmd = new SqlCommand("spGetRegCourses", con);
cmd.CommandType = CommandType.StoredProcedure;
SqlDataAdapter sqlda = new SqlDataAdapter(cmd);
cmd.Parameters.Add(new SqlParameter("@UserID", Uid));
DataSet ds = new DataSet();
sqlda.Fill(ds);
return ds;
}

/BLL
public DataSet StdCourseRegBindBLL(int Uid)
{
return da.StdCourseBindDAL(Uid);
}

/code behind
protected void Page_Load(object sender, EventArgs e)
{
DataSet StudentDetails = new DataSet();
string UName = Session["username"].ToString();
StudentDetails = sbl.GetStudentDetailInLable(UName);
LblS_ID.Text = StudentDetails.Tables[0].Rows[0]["StudentID"].ToString() + "";
LblS_Name.Text = StudentDetails.Tables[0].Rows[0]["StudentName"].ToString() + "";
LblS_eMailID.Text = StudentDetails.Tables[0].Rows[0]["EmailID"].ToString() + "";
LblPhnNum.Text = StudentDetails.Tables[0].Rows[0]["PhnNo"].ToString() + "";
LblS_City.Text = StudentDetails.Tables[0].Rows[0]["city"].ToString() + "
";
}
public void GridBindCourseStdReg(int ID)
{
DataSet bda = sbl.StdCourseRegBindBLL(ID);
GrdCourseStdReg.DataSource = bda;
GrdCourseStdReg.DataBind();
}
protected void BtnCompCourses_Click(object sender, EventArgs e)
{
int UID;
UID =Convert.ToInt32(LblS_ID.Text);
GridBindCourseStdReg(UID);
}