Pull A GUID From Database In Order To Update A Username?

I have a SQL database that is different than the one in ASP.net so I had to create a custom membership Provider. The provider to get the user looks like this:My SQL class name is CustomSqlProvider:

Public Overrides Function GetUser(ByVal username As String, _
ByVal userIsOnline As Boolean) As MembershipUser
'Dim connectionString As String = "Server=***;Database=***;User Id=***password=****"'

[code]....

After adding those gets and sets and assigning parameter variables to my Sproc parameters I get the following error: "Reference not set to an instance of an object." at this line of code:

Dim currentUser As MembershipUser = Membership.GetUser()
Dim UserId As Guid = CType(currentUser.ProviderUserKey, Guid)

And this is what I have on the UserInfo.aspx.vb Page under the Button click:

Dim sql As New SqlClient.SqlCommand("Update_User", con)
sql.Parameters.Add("@UserId", SqlDbType.UniqueIdentifier).Value = UserId
sql.Parameters.Add("@UserName", SqlDbType.NVarChar).Value = txtUserName.Text