diff --git a/src/_zkapauthorizer/tests/test_plugin.py b/src/_zkapauthorizer/tests/test_plugin.py
index 295cc55e38a39b283a119b5a1f407e2244898f45..34025969f5f73f34475933f93a8e2d39705e7f8c 100644
--- a/src/_zkapauthorizer/tests/test_plugin.py
+++ b/src/_zkapauthorizer/tests/test_plugin.py
@@ -20,7 +20,7 @@ from __future__ import (
     absolute_import,
 )
 
-from io import (
+from StringIO import (
     StringIO,
 )
 from os import (
@@ -397,6 +397,11 @@ class ClientPluginTests(TestCase):
             b"tub.port",
             config_text.encode("utf-8"),
         )
+        # On Tahoe-LAFS <1.16, the config is written as bytes.
+        # On Tahoe-LAFS >=1.16, the config is written as unicode.
+        #
+        # After support for Tahoe <1.16 support is dropped we probably want to
+        # switch to an io.StringIO here.
         config_text = StringIO()
         node_config.config.write(config_text)
         self.addDetail(u"config", text_content(config_text.getvalue()))