Oops, don't assume decorator arguments can be hashed.

1 files changed, 1 insertions, 1 deletions

diff --git a/coverage/misc.py b/coverage/misc.py
index 797a16d5..7f85a0eb 100644
--- a/coverage/misc.py
+++ b/coverage/misc.py
@@ -71,7 +71,7 @@ if env.TESTING:
         def _decorator(func):
             argnameset = set(name.strip() for name in argnames.split(","))
             def _wrapped(*args, **kwargs):
-                vals = set(kwargs.get(name) for name in argnameset)
+                vals = [kwargs.get(name) for name in argnameset]
                 assert sum(val is not None for val in vals) == 1
                 return func(*args, **kwargs)
             return _wrapped