diff --git a/src/allmydata/testing/web.py b/src/allmydata/testing/web.py
index 8bd21f7d0..bb858b555 100644
--- a/src/allmydata/testing/web.py
+++ b/src/allmydata/testing/web.py
@@ -104,7 +104,7 @@ def capability_generator(kind):
 
     if kind not in KNOWN_CAPABILITIES:
         raise ValueError(
-            "Unknown capability kind '{} (valid are {})'".format(
+            "Unknown capability kind '{}' (valid are {})".format(
                 kind.decode('ascii'),
                 ", ".join([x.decode('ascii') for x in KNOWN_CAPABILITIES]),
             )