diff -r 1e042e31bd0c -r eca468b8fae4 mercurial/changegroup.py
--- a/mercurial/changegroup.py	Tue Sep 15 17:35:32 2015 -0700
+++ b/mercurial/changegroup.py	Tue Sep 15 17:53:28 2015 -0700
@@ -158,6 +158,8 @@
     deltaheadersize = struct.calcsize(deltaheader)
     version = '01'
     def __init__(self, fh, alg):
+        if alg == 'UN':
+            alg = None # get more modern without breaking too much
         if not alg in util.decompressors:
             raise util.Abort(_('unknown stream compression type: %s')
                              % alg)
@@ -165,7 +167,7 @@
         self._type = alg
         self.callback = None
     def compressed(self):
-        return self._type != 'UN'
+        return self._type is not None
     def read(self, l):
         return self._stream.read(l)
     def seek(self, pos):