主题：自动编号错误提示

自动编号代码

Select e.DataCol.Name
    Case "类别代号"
        If e.DataRow.IsNull("类别代号") Then
            e.DataRow("产品编号") = Nothing
        Else
            Dim lb As String = e.DataRow("类别代号")
            If e.DataRow("产品编号").StartsWith(lb) = False '如果单据产品编号前缀不符
                Dim max As String
                Dim idx As Integer
                max = e.DataTable.Compute("Max(产品编号)","类别代号 = '" & lb & "' And [_Identify] <> " & e.DataRow("_Identify")) '取得该类别代号的最大产品编号
                If max > "" Then '如果存在最大产品编号
                    idx = CInt(max.Substring(2,3)) + 1 '获得最大产品编号的后三位顺序号,并加1
                Else
                    idx = 1 '否则顺序号等于1
                End If
                e.DataRow("产品编号") = lb & Format(idx,"000")
            End If
        End If
End Select

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错误提示！！！！

.NET Framework 版本：2.0.50727.3649
Foxtable 版本：2013.10.14.1
错误所在事件：表，产品表，DataColChanged
详细错误信息：
Exception has been thrown by the target of an invocation.
Index and length must refer to a location within the string.
Parameter name: length