help on checking userID if it exist

hi, im using this code to transfer the username to the login1.username textbox, 

my question is what if the userID doesnt exist in the membership table?

Dim strUserId As String = Request.QueryString("UserId")
Dim userid As System.Guid
userid = New Guid(strUserId.ToString)
Dim u As MembershipUser = Membershi ...