diff --git a/apps/common/tasks.py b/apps/common/tasks.py
index aecc54ca7..eeeee7214 100644
--- a/apps/common/tasks.py
+++ b/apps/common/tasks.py
@@ -24,7 +24,7 @@ def send_mail_async(*args, **kwargs):
     """
     if len(args) == 3:
         args = list(args)
-        args[0] = settings.EMAIL_SUBJECT_PREFIX + args[0]
+        args[0] = (settings.EMAIL_SUBJECT_PREFIX or '') + args[0]
         email_from = settings.EMAIL_FROM or settings.EMAIL_HOST_USER
         args.insert(2, email_from)
         args = tuple(args)