diff --git a/bots/modules/ping.py b/bots/modules/ping.py
index efc95fd..9ecf0db 100644
--- a/bots/modules/ping.py
+++ b/bots/modules/ping.py
@@ -17,15 +17,15 @@ from bots.models import TelegramBotModuleConfig
 class PingBotCategory(models.Model):
     chat_id = models.BigIntegerField(db_index=True)
     cat_name = models.CharField(max_length=64, db_index=True)
-    user_ids = JSONField(default='[]')
+    user_ids = JSONField(null=True)
 
     def add_user(self, uid: int):
-        user_ids = set(self.user_ids)
+        user_ids = set(self.user_ids or [])
         user_ids.add(uid)
         self.user_ids = list(user_ids)
 
     def remove_user(self, uid: int):
-        user_ids = set(self.user_ids)
+        user_ids = set(self.user_ids or [])
         user_ids.add(uid)
         self.user_ids = list(user_ids)