Input string was not in a correct format

Hi,

in my application sometimes this error is coming  when  i click on newbuttonclick or editbuttonclick .I tried  to pass differetn ways but  it shows  same error .sometimes  in first newbtnclick it shows this error.In database EmpID column int.

String userid;
in pageload


 if (Request.QueryString["Id"] != null)
            {
                userid = Request.QueryString["Id"];
                EditControlvalues1();
            }

int userid1 = Convert.ToInt32(userid);
        SqlCommand cmd = new SqlCommand("select EmpName,StartDate,EndDate from EmpTimesheet where EmpId="+@userid1, cn);
        cmd.Parameters.AddWithValue("@EmpId", userid1); ///it shows error

 //cmd.Parameters.AddWithValue("@EmpId", SqlDbType.Int).Value=userid;
     //   cmd.Parameters.AddWithValue("@EmpId", Convert.ToInt32 (userid));