app.config疑问

一服务端应用程序，appconfig有一段配置如下：    <bindings>
      <customBinding>
        <binding name="compactBinding">
          <compactMessageEncoding>
            <binaryMessageEncoding/>
          </compactMessageEncoding>
          <tcpTransport/>
        </binding>
      </customBinding>
    </bindings>
    <extensions>
      <bindingElementExtensions>
        <add name="compactMessageEncoding" type="Amib.WCF.CompactMessageEncodingElement, CompactMessageEncoder, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null"/>
      </bindingElementExtensions>
    </extensions>
当把项目配置-》应用程序:目标framework设为 .net2.0时没有问题
如果设为 .net3.5就会出现错误提示：元素 binding的子元素“compactMessageEncoding”无效。请问这是为什么？
求解，感激不尽！