Casting ProviderUserKey to GUID in VB.NET

I need to get the UserId to pass as a parameter to a stored procedure.  I use VB.NET and none of the sample syntax I have found works.  It always results in an Empty String.

I have tried:
Dim UserId As Object = DirectCast(Membership.GetUser(UserName.Text).ProviderUserKey, Guid)
 and
Dim UserId as GIUD = New GUID(Membership.GetUser(UserName.Text).ProviderUserKey.ToString) Note that the "ProviderUserKey" is being returned (i.e., it is not an empty string!), so I don't know what's wrong with the cast itself. ...