diff -Naur marshmallow-3.1.1.orig/src/marshmallow/schema.py marshmallow-3.1.1/src/marshmallow/schema.py
--- marshmallow-3.1.1.orig/src/marshmallow/schema.py	2019-09-16 14:38:20.000000000 +0200
+++ marshmallow-3.1.1/src/marshmallow/schema.py	2019-09-16 16:19:15.147762565 +0200
@@ -14,7 +14,7 @@
 from marshmallow import base, fields as ma_fields, class_registry
 from marshmallow.error_store import ErrorStore
 from marshmallow.exceptions import ValidationError, StringNotCollectionError
-from marshmallow.orderedset import OrderedSet
+from ordered_set import OrderedSet
 from marshmallow.decorators import (
     POST_DUMP,
     POST_LOAD,