Irreproducible bug fixing attempt

payments/backends/stripe.py

@@ -211,7 +211,10 @@ class StripeBackend(BackendBase):
         payment.paid_amount = payment.amount
         payment.backend_extid = invoice['id']
         if invoice['subscription']:
-            payment.backend_sub_id = invoice['subscription']['id']
+            if isinstance(invoice['subscription'], str):
+                payment.backend_sub_id = invoice['subscription']
+            else:
+                payment.backend_sub_id = invoice['subscription']['id']
         payment.backend_data = {'event_id': event['id']}
         payment.save()