diff --git a/multisite/models.py b/multisite/models.py
index 48b493d..f11cfb5 100644
--- a/multisite/models.py
+++ b/multisite/models.py
@@ -168,11 +168,12 @@ class Alias(models.Model):
     site = models.ForeignKey(
         Site, related_name='aliases', on_delete=models.CASCADE
     )
-    is_canonical = models.NullBooleanField(
+    is_canonical = models.BooleanField(
         _('is canonical?'),
         default=None, editable=False,
         validators=[validate_true_or_none],
         help_text=_('Does this domain name match the one in site?'),
+        null=True, blank=True
     )
     redirect_to_canonical = models.BooleanField(
         _('redirect to canonical?'),