diff --git a/rest_framework/mixins.py b/rest_framework/mixins.py
index 47a4923a1..f3695e665 100644
--- a/rest_framework/mixins.py
+++ b/rest_framework/mixins.py
@@ -71,9 +71,8 @@ class UpdateModelMixin(object):
 
         if getattr(instance, '_prefetched_objects_cache', None):
             # If 'prefetch_related' has been applied to a queryset, we need to
-            # refresh the instance from the database.
-            instance = self.get_object()
-            serializer = self.get_serializer(instance)
+            # forcibly invalidate the prefetch cache on the instance.
+            instance._prefetched_objects_cache = {}
 
         return Response(serializer.data)