diff -r 7568f5c1c801 -r 292331e906d7 mercurial/wireproto.py
--- a/mercurial/wireproto.py	Wed May 28 15:31:05 2014 -0700
+++ b/mercurial/wireproto.py	Tue May 27 10:32:07 2014 -0700
@@ -805,7 +805,8 @@
             os.unlink(tempname)
     except error.BundleValueError, exc:
             bundler = bundle2.bundle20(repo.ui)
-            bundler.newpart('B2X:ERROR:UNKNOWNPART', [('parttype', str(exc))])
+            errpart = bundler.newpart('B2X:ERROR:UNSUPPORTEDCONTENT')
+            errpart.addparam('parttype', str(exc))
             return streamres(bundler.getchunks())
     except util.Abort, inst:
         # The old code we moved used sys.stderr directly.