diff --git a/src/allmydata/test/test_mutable.py b/src/allmydata/test/test_mutable.py
index e6eea1e..c20c0cc 100644
--- a/src/allmydata/test/test_mutable.py
+++ b/src/allmydata/test/test_mutable.py
@@ -3577,3 +3577,26 @@ class Interoperability(GridTestMixin, unittest.TestCase, testutil.ShouldFailMixi
         d = n.download_best_version()
         d.addCallback(self.failUnlessEqual, self.sdmf_old_contents)
         return d
+
+class DifferentEncoding(unittest.TestCase):
+    def setUp(self):
+        self._storage = s = FakeStorage()
+        self.nodemaker = make_nodemaker(s)
+
+    def test_new_encoding(self):
+        # create a file with 3-of-20, then modify it with a client configured
+        # to do 3-of-10. I saw a failure here.
+        self.nodemaker.default_encoding_parameters["n"] = 20
+        d = self.nodemaker.create_mutable_file("old contents")
+        def _created(n):
+            filecap = n.get_cap().to_string()
+            del n # we want a new object, not the cached one
+            self.nodemaker.default_encoding_parameters["n"] = 10
+            n2 = self.nodemaker.create_from_cap(filecap)
+            return n2
+        d.addCallback(_created)
+        def modifier(old_contents, servermap, first_time):
+            return "new contents"
+        d.addCallback(lambda n: n.modify(modifier))
+        return d
+