Cannot find a RadListBox control specified by the TransferTo

I have RadListBox1, and I want to enable drag'n'drop into RadListBox2 that is contained inside a FormView. RadListBox1's properties are set so that TransferToID="RadListBox2". But on execution, I get the error in the title of this post.  I am guessing that it can't find it because it's buried in the FormView.  How can I make this work?