NullReferenceException when setting XML Element Value

I'm working with a simple XML file for generating Excel files. The problem I'm having is editing Worksheet names. Every time I try to update the value, I get a NullReferenceException.

Here's the XML:

<?xml version="1.0"?>
    <?mso-application progid="Excel.Sheet"?>
<Workbook
   xmlns="urn:schemas-microsoft-com:office:spreadsheet"
   xmlns:o="urn:schemas-microsoft-com:office:office"
   xmlns:x="urn:schemas-microsoft-com:office:excel"
   xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet"
   xmlns:html="http://ift.tt/qQdaDR">
  <DocumentProperties xmlns="urn:schemas-microsoft-com:office:office">
  <Author>Me</Author>
  <Company>RMyCo, LLC</Company>
  <Version>1.0</Version>
  </DocumentProperties>
  <ExcelWorkbook xmlns="urn:schemas-microsoft-com:office:excel">
    <WindowHeight>6795</WindowHeight>
    <WindowWidth>8460</WindowWidth>
    <WindowTopX>120</WindowTopX>
    <WindowTopY>15</WindowTopY>
    <ProtectStructure>False</ProtectStructure>
    <ProtectWindows>False</ProtectWindows>
  </ExcelWorkbook>
  <Styles>
    <Style ss:ID="Default" ss:Name="Normal">
      <Alignment ss:Vertical="Bottom" />
      <Borders />
      <Font />
      <Interior />
      <NumberFormat />
      <Protection />
    </Style>
    <Style ss:ID="s21">
      <Font x:Family="Swiss" ss:Bold="1" />
    </Style>
  </Styles>
  <Worksheet ss:Name="Sheet1">
    <Table ss:ExpandedColumnCount="2" ss:ExpandedRowCount="5"
       x:FullColumns="1" x:FullRows="1">
     </Table>
    <WorksheetOptions xmlns="urn:schemas-microsoft-com:office:excel">
      <Print>
        <ValidPrinterInfo />
        <HorizontalResolution>600</HorizontalResolution>
        <VerticalResolution>600</VerticalResolution>
      </Print>
      <Selected />
      <Panes>
        <Pane>
          <Number>3</Number>
          <ActiveRow>5</ActiveRow>
          <ActiveCol>1</ActiveCol>
        </Pane>
      </Panes>
      <Table>
      </Table>
      <ProtectObjects>False</ProtectObjects>
      <ProtectScenarios>False</ProtectScenarios>
    </WorksheetOptions>
  </Worksheet>
</Workbook>

Here's the code:

Imports System
Imports System.Reflection
Imports System.Xml
Imports System.Xml.Linq

Public Class ExcelXML


Public Function RenameSheet(FileNamePath As String, OldSheetName As String, NewSheetName As String) As Integer

        Dim xDoc As New XmlDocument
        Dim xRead As New XmlTextReader(FileNamePath)
        Dim xNode As XmlNode

        xDoc.Load(xRead)

        xNode = xDoc.SelectSingleNode("/Workbook/Worksheet[@Name='Sheet1']")

        xNode.Attributes("Name").Value = NewSheetName

        xDoc.Save(FileNamePath)
        xRead.Close()


        Return 0

    End Function

End Class

It dies whenever I try to rename a sheet. xNode.Attributes("Name").Value = NewSheetName