public void setSortOrder(@Nullable SortOrder sortOrder) {
        if (sortOrder != null && sortOrder.scope != SortOrder.Scope.INTRA_FEED) {
            throw new IllegalArgumentException("The specified sortOrder " + sortOrder
                    + " is invalid. Only those with INTRA_FEED scope are allowed.");
        }
        this.sortOrder = sortOrder;
    }
@Test
    public void testSetSortOrder_NullAllowed() {
        original.setSortOrder(null); // should be okay
    }