我有一个datagrid，其中dataProvider是两种不同类型的对象（FolderVO和FileVO）的ArrayCollection。我有一个size列，在FolderVO的情况下，它由一个名为contentSize的属性填充，在FileVO的情况下，它由size属性填充（差异由itemrenderer处理）。
这意味着我需要为size列实现一个排序函数，如下所示：

protected function sortSize(dataA:Object, dataB:Object):int{
        var order:int = 0;

        if(dataA is FolderVO && dataB is FolderVO){

            order = ObjectUtil.numericCompare(dataA.contentSize, dataB.contentSize);

        }else if(dataA is FileVO && dataB is FileVO){

            order = ObjectUtil.numericCompare(dataA.size, dataB.size);

        }else if(dataA is FolderVO && dataB is FileVO){

            order = 1;

        }else if(dataA is FileVO && dataB is FolderVO){

            order = -1;
        }

        return order;
    }

该函数运行得很好，但在return语句之后，我得到了这个错误：

Error: Find criteria must contain at least one sort field value.
at mx.collections::Sort/findItem()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\Sort.as:491]
at mx.collections::ListCollectionView/getItemIndex()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\ListCollectionView.as:513]
at ListCollectionViewCursor/collectionEventHandler()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\ListCollectionView.as:2154]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.collections::ListCollectionView/dispatchEvent()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\ListCollectionView.as:833]
at mx.collections::ListCollectionView/internalRefresh()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\ListCollectionView.as:1275]
at mx.collections::ListCollectionView/refresh()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\collections\ListCollectionView.as:402]
at mx.controls::DataGrid/sortByColumn()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\controls\DataGrid.as:3560]
at mx.controls::DataGrid/headerReleaseHandler()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\controls\DataGrid.as:4909]
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at mx.core::UIComponent/dispatchEvent()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\core\UIComponent.as:9298]
at mx.controls.dataGridClasses::DataGridHeader/mouseUpHandler()[C:\autobuild\3.2.0\frameworks\projects\framework\src\mx\controls\dataGridClasses\DataGridHeader.as:1259]

正如你所注意到的，错误发生在flex框架本身，而不是我的代码中。所以我真的被困在这里了。帮助将非常感谢。