puzzled! eWasOpenForWrite error

[289924688488291]

hello,everybody!
I get a error 'eWasOpenForWrite' in the code below, can anyone tell me how to tackle this problem?
thanks.

        private static void CheckInference(List<Solid3d> solids) {
            Database db = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument.Database;
            using (Transaction tran = db.TransactionManager.StartTransaction()) {
                BlockTable blockTable = tran.GetObject(db.BlockTableId, OpenMode.ForRead) as BlockTable;
                BlockTableRecord blockRecord = tran.GetObject(blockTable[BlockTableRecord.ModelSpace], OpenMode.ForWrite) as BlockTableRecord;

                for (int i = 0; i < solids.Count - 1; i++) {
                    Solid3d solid1 = solids[i];
                    for (int j = i + 1; j < solids.Count; j++) {
                        Solid3d solid2 = solids[j];
                        if (solid1.Layer != solid2.Layer) {
                            if (solid1.CheckInterference(solid2)) {
                                Solid3d copySolid1 = solid1.Clone() as Solid3d;
                                copySolid1 .BooleanOperation(BooleanOperationType.BoolIntersect, solid2.Clone() as Solid3d);//------eWasOpenForWrite error occured here
                                copySolid1 .ColorIndex = 1;
                                blockRecord.AppendEntity(copySolid1 );
                                tran.AddNewlyCreatedDBObject(copySolid1 , true);
                                tran.Commit();
                            }
                        }
                    }
                }
            }
        }