add test case

counter/models.py

@@ -1402,5 +1402,9 @@ class InvoiceCall(models.Model):
         verbose_name = _("Invoice call")
         verbose_name_plural = _("Invoice calls")
 
+    def save(self, *args, **kwargs):
+        self.month = self._meta.get_field("month").to_python(self.month)
+        super().save(*args, **kwargs)
+
     def __str__(self):
         return f"invoice call of {self.month} made by {self.club}"