Page Load Event - Get UserId from db and put into hidden Label field.

I am working on Scott Mitchell's ASP.Net in 24 hours book. I am running into what looks like a simple problem that is getting the best of me.

When a page loads, the UserId of the currently logged in user will be retrieved in the page Load event and placed into the Text value of a Label. But, when the page opens when I am running the app, I am getting an error in the lower left corner of the browser. When I open up the error message, I see this:
Webpage error details
Message: 'UserIDValue' is undefined
Line: 92
Char: 1
Code: 0
URI: http://localhost:51035/PhotoAlbum24hrs/PhotoAdmin/ManageCategories.aspx
 
Here is the web page's Load event (VB) code:
 
Partial Class PhotoAdmin_Mana ...