Sequence contains no elements at system.linq.enumerable.single[tsource](ienumerable'1 source)

Sequence contains no Elements at System.Linq.Enumerable.Single[Tsource](IEnumerable'1 source)

using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace PopulateContentsOpenXml
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            string myfile = @"C:\Users\Emeka.Iwuagwu\Desktop\DTestDoc.docx";
            string[] writeDocData = new string[] {FirstName.Text,FileNumber.Text,IDNumber.Text,LastName.Text };
            WriteDataToContentControl(myfile, writeDocData);
        }

        private void WriteDataToContentControl(string filename,string[]data)
        {
            try
            {
                using (WordprocessingDocument doc = WordprocessingDocument.Open(filename, true))
                {
                    MainDocumentPart mainPart = doc.MainDocumentPart;
                    foreach (string text in data) 
                    {
                        SdtElement text_block = mainPart.Document.Body.Descendants<SdtElement>().Where(r => r.SdtProperties.GetFirstChild<Tag>().Val == text).Single();
                        Text to = text_block.Descendants<Text>().Single();
                        to.Text = text;
                        mainPart.Document.Save();
                        MessageBox.Show("Ok i am fine now!");
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error: " + ex.ToString());
            }
        }
    }
}

SdtElement text_block = mainPart.Document.Body.Descendants<SdtElement>().Where(r => r.SdtProperties.GetFirstChild<Tag>().Val == text).Single();