VB.NET textbox中只提取14位后的8个值 急。。。

如标题 这是我写的代码。
Private Sub setAttributes()
Dim oSku As String = Session("CreateLoadSKU")
Dim oConsignee As String = Session("CreateLoadConsignee")
Dim objSkuClass As WMS.Logic.SkuClass = New WMS.Logic.SKU(oConsignee, oSku).SKUClass
If Not objSkuClass Is Nothing Then
If objSkuClass.CaptureAtReceivingLoadAttributesCount > 0 Then
For Each oAtt As WMS.Logic.SkuClassLoadAttribute In objSkuClass.LoadAttributes
Dim req As Boolean = False
If oAtt.CaptureAtReceiving = Logic.SkuClassLoadAttribute.CaptureType.Required Then
req = True
End If
If oAtt.CaptureAtReceiving = Logic.SkuClassLoadAttribute.CaptureType.Required Or oAtt.CaptureAtReceiving = Logic.SkuClassLoadAttribute.CaptureType.Capture Then
DO1.AddTextboxLine(oAtt.Name, oAtt.Name)
End If
Dim sButch As String = DO1.Value(oAtt.Name)
Dim substringCode As Boolean = sButch.Substring(14, 8)
Next
Else
End If
Else
End If
End Sub

调试的结果报错.在 Dim substringCode As Boolean = sButch.Substring(14, 8) 这里报错。
错误提示:startIndex 不能大于字符串长度。
我都还没跳转到textbox这个页面就报了这个错误，在next按钮事件中就报错了。谁能帮我修改代码？

先看看sButch.Substring的值是什么，再取字符串。

另外，substringCode定义为布尔类型，赋值语句却是字符串，类型不相符