diff --git a/mercurial/utils/cborutil.py b/mercurial/utils/cborutil.py
--- a/mercurial/utils/cborutil.py
+++ b/mercurial/utils/cborutil.py
@@ -925,6 +925,11 @@
         * Integer number of bytes decoded from the new input.
         * Integer number of bytes wanted to decode the next value.
         """
+        # We /might/ be able to support passing a bytearray all the
+        # way through. For now, let's cheat.
+        if isinstance(b, bytearray):
+            b = bytes(b)
+
         # Our strategy for buffering is to aggregate the incoming chunks in a
         # list until we've received enough data to decode the next item.
         # This is slightly more complicated than using an ``io.BytesIO``
diff --git a/tests/test-cbor.py b/tests/test-cbor.py
--- a/tests/test-cbor.py
+++ b/tests/test-cbor.py
@@ -965,6 +965,14 @@
 
             self.assertEqual(decoder.getavailable(), [source])
 
+    def testbytearray(self):
+        source = b''.join(cborutil.streamencode(b'foobar'))
+
+        decoder = cborutil.bufferingdecoder()
+        decoder.decode(bytearray(source))
+
+        self.assertEqual(decoder.getavailable(), [b'foobar'])
+
 class DecodeallTests(TestCase):
     def testemptyinput(self):
         self.assertEqual(cborutil.decodeall(b''), [])