diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index 7a3491e..d26bdbe 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -427,7 +427,7 @@
             data = {'iat': now, 'exp': expiry, 'iss': self.app_id}
             app_token = jwt.encode(data,
                                    self.app_key,
-                                   algorithm='RS256')
+                                   algorithm='RS256').decode('utf-8')
 
             url = ACCESS_TOKEN_URL % installation_id
             headers = {'Accept': PREVIEW_JSON_ACCEPT,