diff --git a/sendgrid/helpers/mail/mail.py b/sendgrid/helpers/mail/mail.py
index 48d25f161..efeefb8e2 100644
--- a/sendgrid/helpers/mail/mail.py
+++ b/sendgrid/helpers/mail/mail.py
@@ -215,14 +215,14 @@ def get(self):
 
 class Substitution(object):
     def __init__(self, key=None, value=None):
-        self.key = key if key != None else None
-        self.value = value if value != None else None
+        self.key = str(key) if key != None else None
+        self.value = str(value) if value != None else None
 
     def set_key(self, key):
-        self.key = key
+        self.key = str(key) if key != None else None
 
     def set_value(self, value):
-        self.value = value
+        self.value = str(value) if value != None else None
 
     def get(self):
         substitution = {}