diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -753,3 +753,9 @@ def handlereplycaps(op, inpart):
     manargs = dict(inpart.mandatoryparams)
     advargs = dict(inpart.advisoryparams)
     raise util.Abort(manargs['message'], hint=advargs.get('hint'))
+
+@parthandler('b2x:error:unknownpart')
+def handlereplycaps(op, inpart):
+    """Used to transmit unknown part error over the wire"""
+    manargs = dict(inpart.mandatoryparams)
+    raise UnknownPartError(manargs['parttype'])
diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -803,6 +803,12 @@ def unbundle(repo, proto, heads):
         finally:
             fp.close()
             os.unlink(tempname)
+    except bundle2.UnknownPartError, exc:
+            bundler = bundle2.bundle20(repo.ui)
+            part = bundle2.bundlepart('B2X:ERROR:UNKNOWNPART',
+                                      [('parttype', str(exc))])
+            bundler.addpart(part)
+            return streamres(bundler.getchunks())
     except util.Abort, inst:
         # The old code we moved used sys.stderr directly.
         # We did not change it to minimise code change.
diff --git a/tests/test-bundle2.t b/tests/test-bundle2.t
--- a/tests/test-bundle2.t
+++ b/tests/test-bundle2.t
@@ -908,6 +908,8 @@ Setting up
   >     part = None
   >     if reason == 'abort':
   >         part = bundle2.bundlepart('test:abort')
+  >     if reason == 'unknown':
+  >         part = bundle2.bundlepart('TEST:UNKNOWN')
   >     if part is not None:
   >         bundler.addpart(part)
   >     return extradata
@@ -969,3 +971,27 @@ Doing the actual push: Abort error
   [255]
 
 
+Doing the actual push: unknown mandatory parts
+
+  $ cat << EOF >> $HGRCPATH
+  > [failpush]
+  > reason = unknown
+  > EOF
+
+  $ hg -R main push other -r e7ec4e813ba6
+  pushing to other
+  searching for changes
+  abort: missing support for 'test:unknown'
+  [255]
+
+  $ hg -R main push ssh://user@dummy/other -r e7ec4e813ba6
+  pushing to ssh://user@dummy/other
+  searching for changes
+  abort: missing support for "'test:unknown'"
+  [255]
+
+  $ hg -R main push http://localhost:$HGPORT2/ -r e7ec4e813ba6
+  pushing to http://localhost:$HGPORT2/
+  searching for changes
+  abort: missing support for "'test:unknown'"
+  [255]