调用ConstructUnion方法报0x80004005错误

错误信息："System.Runtime.InteropServices.COMException (0x80004005): Error HRESULT E_FAIL has been returned from a call to a COM component. ↵ at ESRI.ArcGIS.Geometry.PolygonClass.ConstructUnion(IEnumGeometry geometries) ↵
 
代码：
ITopologicalOperator pTopo = new Polygon() as ITopologicalOperator;
 pTopo.ConstructUnion(pGeometryCollection as IEnumGeometry);
 
在做ConstructUnion这个方法之前做了简化处理的