Check is cost is not None

aleksis/apps/paweljong/models.py

@@ -312,7 +312,7 @@ class EventRegistration(ExtensibleModel):
     cost = models.IntegerField(verbose_name=_("Cost in €"), null=True, blank=True)
 
     def save(self, *args, **kwargs):
-        if not self.cost:
+        if self.cost is not None:
             self.cost = self.event.cost
         super().save(*args, **kwargs)