--- a/hgext/releasenotes.py	Sun Oct 01 12:29:33 2017 +0100
+++ b/hgext/releasenotes.py	Sun Oct 01 14:37:50 2017 +0100
@@ -576,6 +576,9 @@
 
     incoming = parsenotesfromrevisions(repo, sections.names(), revs)
 
+    if file_ is None:
+        return ui.write(serializenotes(sections, incoming))
+
     try:
         with open(file_, 'rb') as fh:
             notes = parsereleasenotesfile(sections, fh.read())