Exception for add document variable(I cant figure out) - vb.net

Question

I cant figure out where is my error is.Below is my code and the exception:
Imports DocumentFormat.OpenXml
Imports DocumentFormat.OpenXml.Packaging
Imports DocumentFormat.OpenXml.Wordprocessing
Public Class Form1
Public Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
'Create a word file
Using wordDocument As WordprocessingDocument = WordprocessingDocument.Create("C:\Users\local\Desktop\vb testing purpose\test.docx", WordprocessingDocumentType.Document)
' Add a main document part.
Dim mainPart As MainDocumentPart = wordDocument.AddMainDocumentPart()
' Create the document structure and add some text.
mainPart.Document = New Document()
Dim body As Body = mainPart.Document.AppendChild(New Body())
Dim para As Paragraph = body.AppendChild(New Paragraph())
Dim run As Run = para.AppendChild(New Run())
run.AppendChild(New Text(""))
End Using
Dim strDoc As String = "C:\Users\local\Desktop\vb testing purpose\test.docx"
Dim varname As String = "Hi"
Dim varvalue As String = "Bryan"
AddDocVar(strDoc, varname, varvalue)
End Sub
Public Sub AddDocVar(ByVal sender As Object, ByVal varName As String, ByVal varValue As String)
Dim mainPart As MainDocumentPart = sender.MainDocumentPart
Dim settingsPart As DocumentSettingsPart = mainPart.DocumentSettingsPart
settingsPart.Settings = New Settings(New DocumentVariables(New DocumentVariable() With {.Name = varName, .Val = varValue}))
End Sub
End Class
Comes out a
exception(System.MissingMemberException: 'Public member 'MainDocumentPart' on type 'String' not found.') in
Dim mainPart As MainDocumentPart = sender.MainDocumentPart