diff --git a/openwisp_users/base/models.py b/openwisp_users/base/models.py
index b654c173..846f4289 100644
--- a/openwisp_users/base/models.py
+++ b/openwisp_users/base/models.py
@@ -73,7 +73,9 @@ class AbstractUser(BaseUser):
 
     class Meta(BaseUser.Meta):
         abstract = True
-        index_together = ('id', 'email')
+        indexes = [
+        models.Index(fields=['id', 'email']),
+    ]
 
     @staticmethod
     def _get_pk(obj):