ASP.NET TreeView control - CheckedNodes is always empty

I have an ASP.NET TreeView control with the "ShowCheckboxes" state set to "All".

If I check the boxes on the tree, then the tree.CheckedNodes property is always "False". I've also tried looking at the individual Nodes(i).Checked property, but those are also all false.

If I manually set the Checked property to True from code, then it does get reflected in the .CheckedNodes property.

I feel like I must be missing something obvious- why would this simple boolean property fail to reflect what I've done in the UI?

  Protected Function GetChosenIDs() As List(Of Guid)
        Try
            Dim result As List(Of Guid) = New List(Of Guid)

            'This loop never executes, because nothing is marked "Checked".
            For Each node As TreeNode In tree.CheckedNodes
                result.Add(New Guid(node.Value))
         开发者_高级运维   Next
            Return result
        Catch ex As Exception
            Throw ex
        End Try
    End Function