diff --git a/test/unit/cli/test_ringcomposer.py b/test/unit/cli/test_ringcomposer.py
index bacff3ebfc..8183f7f89f 100644
--- a/test/unit/cli/test_ringcomposer.py
+++ b/test/unit/cli/test_ringcomposer.py
@@ -65,7 +65,7 @@ class TestCommands(unittest.TestCase):
         for cmd in cmds:
             try:
                 with open(self.composite_builder_file, 'wb') as fd:
-                    fd.write('not json')
+                    fd.write(b'not json')
                 exit_code, stdout, stderr = self._run_composer(cmd)
                 self.assertEqual(2, exit_code)
                 self.assertIn('An error occurred while loading the composite '
diff --git a/tox.ini b/tox.ini
index a87283b53b..8e2db0e04e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -41,6 +41,7 @@ commands =
       test/unit/cli/test_relinker.py \
       test/unit/cli/test_ring_builder_analyzer.py \
       test/unit/cli/test_ringbuilder.py \
+      test/unit/cli/test_ringcomposer.py \
       test/unit/common/middleware/crypto \
       test/unit/common/middleware/test_catch_errors.py \
       test/unit/common/middleware/test_crossdomain.py \