diff --git a/src/allmydata/test/mutable/test_version.py b/src/allmydata/test/mutable/test_version.py
index 06191b5fe..042305c24 100644
--- a/src/allmydata/test/mutable/test_version.py
+++ b/src/allmydata/test/mutable/test_version.py
@@ -86,7 +86,7 @@ class Version(GridTestMixin, unittest.TestCase, testutil.ShouldFailMixin, \
         def _debug(n):
             fso = debug.FindSharesOptions()
             storage_index = base32.b2a(n.get_storage_index())
-            fso.si_s = storage_index
+            fso.si_s = str(storage_index, "utf-8")  # command-line options are unicode on Python 3
             fso.nodedirs = [os.path.dirname(abspath_expanduser_unicode(str(storedir)))
                             for (i,ss,storedir)
                             in self.iterate_servers()]