diff --git a/sendgrid/helpers/mail/mail.py b/sendgrid/helpers/mail/mail.py
index 2472ad7d..e475fe76 100644
--- a/sendgrid/helpers/mail/mail.py
+++ b/sendgrid/helpers/mail/mail.py
@@ -567,7 +567,7 @@ def add_custom_arg(self, custom_arg):
         :param value: A CustomArg object or a dict of custom arg key/values
         :type value: CustomArg, dict
         """
-        if custom_arg.personalization is not None:
+        if not isinstance(custom_arg, dict) and custom_arg.personalization is not None:
             try:
                 personalization = \
                     self._personalizations[custom_arg.personalization]