diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -831,8 +831,13 @@ def handlereplycaps(op, inpart):
 @parthandler('b2x:error:unsupportedcontent')
 def handlereplycaps(op, inpart):
     """Used to transmit unknown content error over the wire"""
-    parttype = inpart.params['parttype']
-    raise error.BundleValueError(parttype=parttype)
+    kwargs = {}
+    kwargs['parttype'] = inpart.params['parttype']
+    params = inpart.params.get('params')
+    if params is not None:
+        kwargs['params'] = params.split('\0')
+
+    raise error.BundleValueError(**kwargs)
 
 @parthandler('b2x:error:pushraced')
 def handlereplycaps(op, inpart):
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -807,6 +807,8 @@ def unbundle(repo, proto, heads):
             bundler = bundle2.bundle20(repo.ui)
             errpart = bundler.newpart('B2X:ERROR:UNSUPPORTEDCONTENT')
             errpart.addparam('parttype', exc.parttype)
+            if exc.params:
+                errpart.addparam('params', '\0'.join(exc.params))
             return streamres(bundler.getchunks())
     except util.Abort, inst:
         # The old code we moved used sys.stderr directly.