diff --git a/auditlog/models.py b/auditlog/models.py
index 066fe1c..b13763a 100644
--- a/auditlog/models.py
+++ b/auditlog/models.py
@@ -366,7 +366,11 @@ class LogEntry(models.Model):
         verbose_name=_("actor"),
     )
     cid = models.CharField(
-        max_length=255, db_index=True, blank=True, verbose_name=_("Correlation ID")
+        max_length=255,
+        db_index=True,
+        blank=True,
+        null=True,
+        verbose_name=_("Correlation ID"),
     )
     remote_addr = models.GenericIPAddressField(
         blank=True, null=True, verbose_name=_("remote address")