-                child_book = Book.objects.get(slug=slug)
-                child_book.parent = book
-                child_book.parent_number = n
-                child_book.save()
-
+                try:
+                    child_book = Book.objects.get(slug=slug)
+                    child_book.parent = book
+                    child_book.parent_number = n
+                    child_book.save()
+                except Book.DoesNotExist, e:
+                    raise Book.DoesNotExist(u'Book with slug = "%s" does not exist.' % slug)
+