--- a/mercurial/wireproto.py	Tue May 27 10:32:07 2014 -0700
+++ b/mercurial/wireproto.py	Wed May 28 15:51:19 2014 -0700
@@ -806,7 +806,7 @@
     except error.BundleValueError, exc:
             bundler = bundle2.bundle20(repo.ui)
             errpart = bundler.newpart('B2X:ERROR:UNSUPPORTEDCONTENT')
-            errpart.addparam('parttype', str(exc))
+            errpart.addparam('parttype', exc.parttype)
             return streamres(bundler.getchunks())
     except util.Abort, inst:
         # The old code we moved used sys.stderr directly.