--- a/mercurial/bundle2.py	Tue May 27 12:16:45 2014 -0700
+++ b/mercurial/bundle2.py	Wed May 28 16:46:58 2014 -0700
@@ -840,7 +840,9 @@
 def handlereplycaps(op, inpart):
     """Used to transmit unknown content error over the wire"""
     kwargs = {}
-    kwargs['parttype'] = inpart.params['parttype']
+    parttype = inpart.params.get('parttype')
+    if parttype is not None:
+        kwargs['parttype'] = parttype
     params = inpart.params.get('params')
     if params is not None:
         kwargs['params'] = params.split('\0')

--- a/mercurial/error.py	Tue May 27 12:16:45 2014 -0700
+++ b/mercurial/error.py	Wed May 28 16:46:58 2014 -0700
@@ -102,10 +102,13 @@
 class BundleValueError(ValueError):
     """error raised when bundle2 cannot be processed"""
 
-    def __init__(self, parttype, params=()):
+    def __init__(self, parttype=None, params=()):
         self.parttype = parttype
         self.params = params
-        msg = parttype
+        if self.parttype is None:
+            msg = 'Stream Parameter'
+        else:
+            msg = parttype
         if self.params:
             msg = '%s - %s' % (msg, ', '.join(self.params))
         super(BundleValueError, self).__init__(msg)

--- a/mercurial/wireproto.py	Tue May 27 12:16:45 2014 -0700
+++ b/mercurial/wireproto.py	Wed May 28 16:46:58 2014 -0700
@@ -806,7 +806,8 @@
     except error.BundleValueError, exc:
             bundler = bundle2.bundle20(repo.ui)
             errpart = bundler.newpart('B2X:ERROR:UNSUPPORTEDCONTENT')
-            errpart.addparam('parttype', exc.parttype)
+            if exc.parttype is not None:
+                errpart.addparam('parttype', exc.parttype)
             if exc.params:
                 errpart.addparam('params', '\0'.join(exc.params))
             return streamres(bundler.getchunks())