diff -r d618558e4e8b -r 326b174c6a47 mercurial/bundle2.py
--- a/mercurial/bundle2.py	Tue May 08 11:39:38 2018 +0200
+++ b/mercurial/bundle2.py	Tue May 08 14:17:46 2018 -0700
@@ -1659,7 +1659,8 @@
             for n in sorted(closed):
                 yield n
 
-    bundler.newpart('cache:rev-branch-cache', data=generate())
+    bundler.newpart('cache:rev-branch-cache', data=generate(),
+                    mandatory=False)
 
 def _formatrequirementsspec(requirements):
     return urlreq.quote(','.join(sorted(requirements)))