diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -828,9 +828,9 @@ def handlereplycaps(op, inpart):
     """Used to transmit abort error over the wire"""
     raise util.Abort(inpart.params['message'], hint=inpart.params.get('hint'))
 
-@parthandler('b2x:error:unknownpart')
+@parthandler('b2x:error:unsupportedcontent')
 def handlereplycaps(op, inpart):
-    """Used to transmit unknown part error over the wire"""
+    """Used to transmit unknown content error over the wire"""
     raise error.BundleValueError(inpart.params['parttype'])
 
 @parthandler('b2x:error:pushraced')
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -805,7 +805,8 @@ def unbundle(repo, proto, heads):
             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.