diff --git a/blti/__init__.py b/blti/__init__.py
index bcd12a1..6092a53 100644
--- a/blti/__init__.py
+++ b/blti/__init__.py
@@ -44,7 +44,7 @@ def filter_oauth_params(self, params):
 
     def _encrypt_session(self, data):
         aes = aes128cbc(settings.BLTI_AES_KEY, settings.BLTI_AES_IV)
-        return b64encode(aes.encrypt(json.dumps(data)))
+        return b64encode(aes.encrypt(json.dumps(data))).decode('utf8')
 
     def _decrypt_session(self, string):
         aes = aes128cbc(settings.BLTI_AES_KEY, settings.BLTI_AES_IV)