diff --git a/model_utils/models.py b/model_utils/models.py
index fdf13a2..da7e879 100644
--- a/model_utils/models.py
+++ b/model_utils/models.py
@@ -34,8 +34,12 @@ class TimeStampedModel(models.Model):
         modified field is updated even if it is not given as
         a parameter to the update field argument.
         """
-        if 'update_fields' in kwargs and 'modified' not in kwargs['update_fields']:
-            kwargs['update_fields'] += ['modified']
+        update_fields = kwargs.get('update_fields', None)
+        if update_fields is not None:
+            update_fields = set(update_fields)
+            if update_fields:
+                kwargs['update_fields'] = update_fields.union({'modified'})
+
         super().save(*args, **kwargs)
 
     class Meta: