diff --git a/src/calibre/db/legacy.py b/src/calibre/db/legacy.py
index acab647f07..150edf9c0a 100644
--- a/src/calibre/db/legacy.py
+++ b/src/calibre/db/legacy.py
@@ -20,7 +20,7 @@ from calibre.db.cache import Cache
 from calibre.db.errors import NoSuchFormat
 from calibre.db.categories import CATEGORY_SORTS
 from calibre.db.view import View
-from calibre.db.write import clean_identifier
+from calibre.db.write import clean_identifier, get_series_values
 from calibre.utils.date import utcnow
 from calibre.utils.search_query_parser import set_saved_searches
 
@@ -645,8 +645,10 @@ class LibraryDatabase(object):
             else:
                 affected_books = self.new_api._set_field(field, {book_id:val}, allow_case_change=allow_case_change)
             if data['datatype'] == 'series':
-                extra = 1.0 if extra is None else extra
-                self.new_api._set_field(field + '_index', {book_id:extra})
+                s, sidx = get_series_values(val)
+                if sidx is None:
+                    extra = 1.0 if extra is None else extra
+                    self.new_api._set_field(field + '_index', {book_id:extra})
         if notify and affected_books:
             self.notify('metadata', list(affected_books))
         return affected_books
diff --git a/src/calibre/db/tests/legacy.py b/src/calibre/db/tests/legacy.py
index 6ffb7f987d..2387e7f35f 100644
--- a/src/calibre/db/tests/legacy.py
+++ b/src/calibre/db/tests/legacy.py
@@ -760,6 +760,12 @@ class LegacyTest(BaseTest):
         ndb.delete_custom_column('created')
         ndb = self.init_legacy(n)
         self.assertRaises(KeyError, ndb.custom_field_name, num=num)
+
+        # Test setting custom series
+        ndb = self.init_legacy(self.cloned_library)
+        ndb.set_custom(1, 'TS [9]', label='series')
+        self.assertEqual(ndb.new_api.field_for('#series', 1), 'TS')
+        self.assertEqual(ndb.new_api.field_for('#series_index', 1), 9)
     # }}}
 
     def test_legacy_original_fmt(self):  # {{{