diff --git a/src/wormhole/server/server.py b/src/wormhole/server/server.py
index bddf4a4..6b2419f 100644
--- a/src/wormhole/server/server.py
+++ b/src/wormhole/server/server.py
@@ -131,6 +131,7 @@ class RelayServer(service.MultiService):
         log.msg("get_stats took:", time.time() - start)
 
         with open(tmpfn, "wb") as f:
-            json.dump(data, f, indent=1)
-            f.write("\n")
+            # json.dump(f) has str-vs-unicode issues on py2-vs-py3
+            f.write(json.dumps(data, indent=1).encode("utf-8"))
+            f.write(b"\n")
         os.rename(tmpfn, self._stats_file)
diff --git a/src/wormhole/test/test_server.py b/src/wormhole/test/test_server.py
index c79ebf0..89092b1 100644
--- a/src/wormhole/test/test_server.py
+++ b/src/wormhole/test/test_server.py
@@ -1,5 +1,5 @@
 from __future__ import print_function, unicode_literals
-import json, itertools, time
+import os, json, itertools, time
 from binascii import hexlify
 import mock
 from twisted.trial import unittest
@@ -11,7 +11,7 @@ from twisted.web import client
 from autobahn.twisted import websocket
 from .. import __version__
 from .common import ServerBase
-from ..server import rendezvous, transit_server
+from ..server import server, rendezvous, transit_server
 from ..server.rendezvous import Usage, SidedMessage
 from ..server.database import get_db
 
@@ -1043,6 +1043,29 @@ class Summary(unittest.TestCase):
         row = db.execute("SELECT * FROM `mailbox_usage`").fetchone()
         self.assertEqual(row["started"], 20)
 
+class DumpStats(unittest.TestCase):
+    def test_nostats(self):
+        rs = server.RelayServer(str("tcp:0"), str("tcp:0"), None)
+        # with no ._stats_file, this should do nothing
+        rs.dump_stats(1, 1)
+
+    def test_empty(self):
+        basedir = self.mktemp()
+        os.mkdir(basedir)
+        fn = os.path.join(basedir, "stats.json")
+        rs = server.RelayServer(str("tcp:0"), str("tcp:0"), None,
+                                stats_file=fn)
+        now = 1234
+        validity = 500
+        rs.dump_stats(now, validity)
+        with open(fn, "rb") as f:
+            data_bytes = f.read()
+        data = json.loads(data_bytes.decode("utf-8"))
+        self.assertEqual(data["created"], now)
+        self.assertEqual(data["valid_until"], now+validity)
+        self.assertEqual(data["rendezvous"]["all_time"]["mailboxes_total"], 0)
+        self.assertEqual(data["transit"]["all_time"]["total"], 0)
+
 
 class Accumulator(protocol.Protocol):
     def __init__(self):