主题：列集合

Dim cls() As String = {"学段","学科"}  ‘列集合
Dim t As Table = Tables("课程设置")
    For i As Integer = 0 To cls.Length - 1
        If t.Rows(cls(i)) Is Nothing Then

            MessageBox.Show("【" & cls(i) & "】内容不能为空!")
            t.Rows(cls(i)).Select()
            Exit For
            Return
        End If
    Next

请大师指点，问题出在哪?