diff --git a/src/allmydata/test/web/test_web.py b/src/allmydata/test/web/test_web.py
index 1908afdeb..e975464d3 100644
--- a/src/allmydata/test/web/test_web.py
+++ b/src/allmydata/test/web/test_web.py
@@ -746,7 +746,10 @@ class MultiFormatResourceTests(TrialTestCase):
             "<title>400 - Bad Format</title>", response_body,
         )
         self.assertIn(
-            "Unknown t value: 'foo'", response_body,
+            "Unknown t value:", response_body,
+        )
+        self.assertIn(
+            "'foo'", response_body,
         )
 
 
diff --git a/src/allmydata/web/common.py b/src/allmydata/web/common.py
index 2832cc6a8..57118d1d4 100644
--- a/src/allmydata/web/common.py
+++ b/src/allmydata/web/common.py
@@ -1,5 +1,5 @@
 from past.builtins import unicode
-from six import ensure_text
+from six import ensure_text, ensure_str
 
 import time
 import json
@@ -112,7 +112,7 @@ def parse_replace_arg(replace):
     try:
         return boolean_of_arg(replace)
     except WebError:
-        raise WebError("invalid replace= argument: %r" % (replace,), http.BAD_REQUEST)
+        raise WebError("invalid replace= argument: %r" % (ensure_str(replace),), http.BAD_REQUEST)
 
 
 def get_format(req, default="CHK"):