diff --git a/CHANGES b/CHANGES
index 74481b4d3..23c99e9a0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -29,6 +29,7 @@ Bugs fixed
 * utils package is no longer installed
 * #3952: apidoc: module header is too escaped
 * #4275: Formats accepted by sphinx.util.i18n.format_date are limited
+* #4493: recommonmark raises AttributeError if AutoStructify enabled
 
 Testing
 --------
diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 17f9667a1..8f880f88b 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -59,7 +59,9 @@ default_settings = {
     'embed_stylesheet': False,
     'cloak_email_addresses': True,
     'pep_base_url': 'https://www.python.org/dev/peps/',
+    'pep_references': None,
     'rfc_base_url': 'https://tools.ietf.org/html/',
+    'rfc_references': None,
     'input_encoding': 'utf-8-sig',
     'doctitle_xform': False,
     'sectsubtitle_xform': False,