complex databind question

[bprego]

I am getting this error and I dont know why.

An unhandled exception of type 'System.InvalidOperationException' occurred in system.windows.forms.dll
Additional information: Cannot create a child list for field ?????.
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Thanks in advance for any help.
this is my code:

varAccountID = lblAccountID.Text
        'fills demos dataset        
        Dim selectCMD As SqlCommand = New SqlCommand("SELECT q.question_id, q.short_form, '' FROM question AS q INNER JOIN question_set AS qs ON q.question_id=qs.question_id WHERE qs.affinity_id=1 AND q.question_id NOT IN (SELECT v.question_id FROM answer_heading AS h INNER JOIN answer_value AS v ON h.answer_id=v.answer_id WHERE h.account_id = '" & varAccountID & "')UNION SELECT q.question_id, q.short_form, v.response FROM answer_heading AS h INNER JOIN answer_value AS v ON h.answer_id=v.answer_id INNER JOIN question AS q ON v.question_id=q.question_id WHERE(h.account_id = '" & varAccountID & "')ORDER BY q.question_id", SqlConnection1)

        Dim daAnswers As SqlDataAdapter = New SqlDataAdapter
        daAnswers.SelectCommand = selectCMD

        SqlConnection1.Open()

        Dim DsAnswers1 As DataSet = New DataSet
        DataGrid1.DataSource = DsAnswers1

        DsAnswers1.Clear()
        daAnswers.Fill(DsAnswers1)

        SqlConnection1.Close()