Specified cast error - asp.net

Question

I am using VS2012 with ASP.NET 4.5 and MySQL as my database provider.
I am getting a specified cast error now on code that was working fine at one point.
It is from my Register.aspx page for registration verification and the code is adapted from an asp.net website tutorial.
Here is the code, the error is on the Dim newUser as Guid line
Protected Sub RegisterUser_CreatedUser(ByVal sender As Object, ByVal e As EventArgs) Handles RegisterUser.CreatedUser
FormsAuthentication.SetAuthCookie(RegisterUser.UserName, False)
Dim newUser As MembershipUser = Membership.GetUser(RegisterUser.UserName)
Dim newUserID As Guid = DirectCast(newUser.ProviderUserKey, Guid)
Dim urlBase As String = Request.Url.GetLeftPart(UriPartial.Authority) & Request.ApplicationPath
Dim verifyUrl As String = "VerifyNewUser.aspx?ID=" & newUserID.ToString()
Dim fullPath As String = urlBase & verifyUrl
Dim appPath As String = Request.PhysicalApplicationPath
Dim sr As New StreamReader(appPath & "EmailTemplates/VerifyUserAccount.txt")
Dim mailMessage As New MailMessage()
mailMessage.IsBodyHtml = True
mailMessage.From = New MailAddress("myacct#gmail.com")
mailMessage.To.Add(New MailAddress(RegisterUser.Email))
mailMessage.CC.Add(New MailAddress("myacct#gmail.com"))
mailMessage.Subject = "New User Registration"
mailMessage.Body = sr.ReadToEnd
sr.Close()
mailMessage.Body = mailMessage.Body.Replace("<%UserName%>", RegisterUser.UserName)
mailMessage.Body = mailMessage.Body.Replace("<%VerificationUrl%>", fullPath)
//Set up the smtp for gmail to send the email
Dim mailClient As New SmtpClient()
With mailClient
.Port = 587 'try 465 if this doesn't work
.EnableSsl = True
.DeliveryMethod = SmtpDeliveryMethod.Network
.UseDefaultCredentials = False
.Credentials = New NetworkCredential(mailMessage.From.ToString(), "password")
.Host = "smtp.gmail.com"
End With
mailClient.Send(mailMessage)
Dim continueUrl As String = RegisterUser.ContinueDestinationPageUrl
If String.IsNullOrEmpty(continueUrl) Then
continueUrl = "~/"
End If
Response.Redirect(continueUrl)
End Sub

Answer 1

When you, or anyone, creates a MembershipProvider you need to specify whether the ProviderUserKey is going to be an integer or a Guid. The default SqlMembershipProvider implements it a a Guid while the default MySqlMembershipProvider implements it as an int.
You could always implement your own provider by inheriting from one of the defaults and implementing your own version of the ProviderUserKey