diff --git a/newsfragments/3675.minor b/newsfragments/3675.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/test/__init__.py b/src/allmydata/test/__init__.py
index c75f8d003..e9c47bd69 100644
--- a/src/allmydata/test/__init__.py
+++ b/src/allmydata/test/__init__.py
@@ -36,7 +36,7 @@ from foolscap.logging.incident import IncidentQualifier
 if PY3:
     # Error on BytesWarnings, to catch things like str(b""), but only for
     # allmydata code.
-    warnings.filterwarnings("error", category=BytesWarning, module="allmydata.*")
+    warnings.filterwarnings("error", category=BytesWarning, module=".*allmydata.*")
 
 
 class NonQualifier(IncidentQualifier, object):
diff --git a/src/allmydata/test/test_download.py b/src/allmydata/test/test_download.py
index 3a42b0819..d61942839 100644
--- a/src/allmydata/test/test_download.py
+++ b/src/allmydata/test/test_download.py
@@ -1304,7 +1304,7 @@ class MyShare(object):
         self._dyhb_rtt = rtt
 
     def __repr__(self):
-        return "sh%d-on-%s" % (self._shnum, self._server.get_name())
+        return "sh%d-on-%s" % (self._shnum, str(self._server.get_name(), "ascii"))
 
 class MySegmentFetcher(SegmentFetcher):
     def __init__(self, *args, **kwargs):
@@ -1383,7 +1383,7 @@ class Selection(unittest.TestCase):
             self.failUnless(node.failed)
             self.failUnless(node.failed.check(NotEnoughSharesError))
             sname = serverA.get_name()
-            self.failUnlessIn("complete= pending=sh0-on-%s overdue= unused="  % sname,
+            self.failUnlessIn("complete= pending=sh0-on-%s overdue= unused="  % str(sname, "ascii"),
                               str(node.failed))
         d.addCallback(_check2)
         return d
@@ -1605,7 +1605,7 @@ class Selection(unittest.TestCase):
             self.failUnless(node.failed)
             self.failUnless(node.failed.check(NotEnoughSharesError))
             sname = servers[b"peer-2"].get_name()
-            self.failUnlessIn("complete=sh0 pending= overdue=sh2-on-%s unused=" % sname,
+            self.failUnlessIn("complete=sh0 pending= overdue=sh2-on-%s unused=" % str(sname, "ascii"),
                               str(node.failed))
         d.addCallback(_check4)
         return d
diff --git a/src/allmydata/test/test_runner.py b/src/allmydata/test/test_runner.py
index f6a7c2ee1..c80b5dc9c 100644
--- a/src/allmydata/test/test_runner.py
+++ b/src/allmydata/test/test_runner.py
@@ -515,7 +515,7 @@ class RunNode(common_util.SignalMixin, unittest.TestCase, pollmixin.PollMixin):
             0,
             "Expected error message from '{}', got something else: {}".format(
                 description,
-                p.get_buffered_output(),
+                str(p.get_buffered_output(), "utf-8"),
             ),
         )