diff --git a/flashcards/models.py b/flashcards/models.py
index f5ecaa4..b5cd1a7 100644
--- a/flashcards/models.py
+++ b/flashcards/models.py
@@ -99,6 +99,8 @@ class User(AbstractUser, SimpleEmailConfirmationUserMixin):
                   "noreply@flashy.cards", [self.email])
 
     def notify(self):
+        if self.registration_id is None:
+            raise ValueError("user's registration_id is null")
         gcm_ctx = gcm.GCM(GCM_API_KEY)
         try:
             gcm_ctx.plaintext_request(