Send "parentID" with "null" value on dataSource update requets

Hello,

 My model is

 model: {
                    id: 'id',
                    fields: {
                        id: {type: 'number'},
                        parentId: {field: "parentId", nullable: true},
                       // some other fields
                    }
                }

When I edit item from my datasource and call dataSource.sync()  in dataSource update request value of parentId parameter is empty. Than when I receive the response with parentId: "" treelistview deletes the corresponding row.

How can I manage to send parentId with null value to server?

Thanks in advance