diff --git a/src/allmydata/immutable/downloader/node.py b/src/allmydata/immutable/downloader/node.py
index 10ce0e5c7..02153444a 100644
--- a/src/allmydata/immutable/downloader/node.py
+++ b/src/allmydata/immutable/downloader/node.py
@@ -49,6 +49,8 @@ class DownloadNode(object):
     """Internal class which manages downloads and holds state. External
     callers use CiphertextFileNode instead."""
 
+    default_max_segment_size = DEFAULT_MAX_SEGMENT_SIZE
+    
     # Share._node points to me
     def __init__(self, verifycap, storage_broker, secret_holder,
                  terminator, history, download_status):
@@ -76,7 +78,7 @@ class DownloadNode(object):
         # .guessed_segment_size, .guessed_num_segments, and
         # .ciphertext_hash_tree (with a dummy, to let us guess which hashes
         # we'll need)
-        self._build_guessed_tables(DEFAULT_MAX_SEGMENT_SIZE)
+        self._build_guessed_tables(self.default_max_segment_size)
 
         # filled in when we parse a valid UEB
         self.have_UEB = False
diff --git a/src/allmydata/test/test_system.py b/src/allmydata/test/test_system.py
index 10a64c1fe..235565020 100644
--- a/src/allmydata/test/test_system.py
+++ b/src/allmydata/test/test_system.py
@@ -28,13 +28,16 @@ from allmydata.storage.server import si_a2b
 from allmydata.immutable import offloaded, upload
 from allmydata.immutable.literal import LiteralFileNode
 from allmydata.immutable.filenode import ImmutableFileNode
+from allmydata.immutable.downloader.node import DownloadNode
 from allmydata.util import idlib, mathutil
 from allmydata.util import log, base32
 from allmydata.util.encodingutil import quote_output, unicode_to_argv
 from allmydata.util.fileutil import abspath_expanduser_unicode
 from allmydata.util.consumer import MemoryConsumer, download_to_data
+from allmydata.util.deferredutil import async_to_deferred
 from allmydata.interfaces import IDirectoryNode, IFileNode, \
-     NoSuchChildError, NoSharesError, SDMF_VERSION, MDMF_VERSION
+     NoSuchChildError, NoSharesError, SDMF_VERSION, MDMF_VERSION, \
+     DEFAULT_MAX_SEGMENT_SIZE
 from allmydata.monitor import Monitor
 from allmydata.mutable.common import NotWriteableError
 from allmydata.mutable import layout as mutable_layout
@@ -1811,6 +1814,46 @@ class SystemTest(SystemTestMixin, RunBinTahoeMixin, unittest.TestCase):
         d.addCallback(_got_lit_filenode)
         return d
 
+    @async_to_deferred
+    async def test_upload_download_immutable_different_default_max_segment_size(self):
+        """
+        Tahoe-LAFS used to have a default max segment size of 128KB, and is now
+        1MB.  Test that an upload created when 128KB was the default can be
+        downloaded with 1MB as the default (i.e. old uploader, new downloader),
+        and vice versa, (new uploader, old downloader).
+        """
+        await self.set_up_nodes(2)
+
+        # Just 1 share:
+        for c in self.clients:
+            c.encoding_params["k"] = 1
+            c.encoding_params["happy"] = 1
+            c.encoding_params["n"] = 1
+
+        await self._upload_download_different_max_segment(128 * 1024, 1024 * 1024)
+        
+        await self._upload_download_different_max_segment(1024 * 1024, 128 * 1024)
+
+
+    async def _upload_download_different_max_segment(
+            self, upload_segment_size, download_segment_size
+    ):
+        """Upload with one max segment size, download with another."""
+        data = b"123456789" * 1_000_000
+
+        uploader = self.clients[0].getServiceNamed("uploader")
+        uploadable = upload.Data(data, convergence=None)
+        assert uploadable.max_segment_size == None
+        uploadable.max_segment_size = upload_segment_size
+        results = await uploader.upload(uploadable)
+
+        assert DownloadNode.default_max_segment_size == DEFAULT_MAX_SEGMENT_SIZE
+        self.patch(DownloadNode, "default_max_segment_size", download_segment_size)
+        uri = results.get_uri()
+        node = self.clients[1].create_node_from_uri(uri)
+        mc = await node.read(MemoryConsumer(), 0, None)
+        self.assertEqual(b"".join(mc.chunks), data)
+
 
 class Connections(SystemTestMixin, unittest.TestCase):
     FORCE_FOOLSCAP_FOR_STORAGE = True