VB.Net signedXml "Invalid character in a Base-64 string" - vb.net

Question

I'm getting an error everytime I try to upload a XML file to an specific server.
It returns "Invalid character in a Base-64 string". Here the code I'm using to sign:
Public Sub Assinar03(ByVal strArqXMLAssinar As String, ByVal strUri As String, ByVal x509Certificado As X509Certificate2, ByVal strArqXMLAssinado As String)
Dim SR As StreamReader = Nothing
SR = File.OpenText(strArqXMLAssinar)
Dim vXMLString As String = SR.ReadToEnd()
SR.Close()
Dim _xnome As String = String.Empty
Dim _serial As String = String.Empty
If x509Certificado IsNot Nothing Then
_xnome = x509Certificado.Subject.ToString()
_serial = x509Certificado.SerialNumber
End If
Dim _X509Cert As New X509Certificate2()
Dim store As New X509Store("MY", StoreLocation.CurrentUser)
store.Open(OpenFlags.[ReadOnly] Or OpenFlags.OpenExistingOnly)
Dim collection As X509Certificate2Collection = DirectCast(store.Certificates, X509Certificate2Collection)
Dim collection1 As X509Certificate2Collection = DirectCast(collection.Find(X509FindType.FindBySerialNumber, _serial, False), X509Certificate2Collection)
If collection1.Count > 0 Then
_X509Cert = Nothing
For i As Integer = 0 To collection1.Count - 1
If DateTime.Now < collection1(i).NotAfter OrElse Not _X509Cert Is Nothing AndAlso _X509Cert.NotAfter < collection1(i).NotAfter Then
_X509Cert = collection1(i)
End If
Next
If _X509Cert Is Nothing Then _X509Cert = collection1(0)
Dim doc As New XmlDocument()
doc.PreserveWhitespace = False
doc.LoadXml(vXMLString)
Dim qtdeRefUri As Integer = doc.GetElementsByTagName(strUri).Count
Dim reference As New Reference()
Dim keyInfo As New KeyInfo()
Dim signedXml As New SignedXml(doc)
signedXml.SigningKey = _X509Cert.PrivateKey
Dim _Uri As XmlAttributeCollection = doc.GetElementsByTagName(strUri).Item(0).Attributes
For Each _atributo As XmlAttribute In _Uri
If _atributo.Name.ToLower.Trim = "Id".ToLower.Trim Then
reference.Uri = "#" + _atributo.InnerText
End If
Next
If reference.Uri Is Nothing Then reference.Uri = ""
reference.DigestMethod = SignedXml.XmlDsigSHA1Url
'--------------------------------------------------
Dim env As New XmlDsigEnvelopedSignatureTransform()
env.Algorithm = "http://www.w3.org/2000/09/xmldsig#enveloped-signature"
reference.AddTransform(env)
'--------------------------
Dim c14 As New XmlDsigC14NTransform(False)
c14.Algorithm = "http://www.w3.org/TR/2001/REC-xml-c14n-20010315"
reference.AddTransform(c14)
'--------------------------
signedXml.AddReference(reference)
keyInfo.AddClause(New KeyInfoX509Data(_X509Cert))
'--------------------------
signedXml.KeyInfo = keyInfo
signedXml.ComputeSignature()
'--
Dim xmlDigitalSignature As XmlElement = signedXml.GetXml()
doc.DocumentElement.AppendChild(doc.ImportNode(xmlDigitalSignature, True))
XMLDoc = New XmlDocument()
XMLDoc.PreserveWhitespace = False
XMLDoc = doc
Me.vXMLStringAssinado = XMLDoc.OuterXml
'-----------
Dim SW_2 As StreamWriter = File.CreateText(strArqXMLAssinado)
SW_2.Write(Me.vXMLStringAssinado)
SW_2.Close()
'-----------
End If
SR.Close()
End Sub
Is there something else I should add to the code?
The manual tells me to follow the instructions from https://www.w3.org/TR/xmldsig-core/

Answer 1

Turns out it was a line break when saving the document. I set the .PreserveWhitespace property to true before saving the .xml file and not it seems to be working.