diff --git a/longs/models.py b/longs/models.py
index 74f2cdd6..624acb23 100644
--- a/longs/models.py
+++ b/longs/models.py
@@ -15,6 +15,8 @@ from django.urls import reverse
 from django.utils.translation import ugettext_lazy as _
 from django.utils.html import mark_safe
 
+from django.core.exceptions import ValidationError
+
 from sortedm2m.fields import SortedManyToManyField
 
 import string