diff --git a/src/allmydata/test/test_system.py b/src/allmydata/test/test_system.py
index e72d8392e..048ab3539 100644
--- a/src/allmydata/test/test_system.py
+++ b/src/allmydata/test/test_system.py
@@ -1077,7 +1077,7 @@ class SystemTest(testutil.SignalMixin, unittest.TestCase):
             tdir = self.getdir("cli_put")
             fileutil.make_dirs(tdir)
             fn = os.path.join(tdir, "upload_me")
-            f = open(fn, "w")
+            f = open(fn, "wb")
             f.write(TESTDATA)
             f.close()
             argv = ["put"] + nodeargs + [fn, "test_put/upload.txt"]