diff --git a/src/allmydata/test/common_py3.py b/src/allmydata/test/common_py3.py
index b29cb9691..157f22fdf 100644
--- a/src/allmydata/test/common_py3.py
+++ b/src/allmydata/test/common_py3.py
@@ -9,7 +9,7 @@ from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
 
-from future.utils import PY2
+from future.utils import PY2, native_str, native_bytes
 if PY2:
     from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, list, object, range, str, max, min  # noqa: F401
 
@@ -72,7 +72,7 @@ class ShouldFailMixin(object):
 
     def shouldFail(self, expected_failure, which, substring,
                    callable, *args, **kwargs):
-        assert substring is None or isinstance(substring, str)
+        assert substring is None or isinstance(substring, (native_str, native_bytes))
         d = defer.maybeDeferred(callable, *args, **kwargs)
         def done(res):
             if isinstance(res, failure.Failure):