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

2 files changed, 3 insertions, 3 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
diff --git a/tests/test_misc.py b/tests/test_misc.py
index fc95744a..e69c2cdd 100644
--- a/tests/test_misc.py
+++ b/tests/test_misc.py
@@ -102,10 +102,10 @@ class ContractTest(CoverageTest):
             return (a, b, c)
 
         assert give_me_one(a=17) == (17, None, None)
-        assert give_me_one(b=17) == (None, 17, None)
+        assert give_me_one(b=set()) == (None, set(), None)
         assert give_me_one(c=17) == (None, None, 17)
         with pytest.raises(AssertionError):
-            give_me_one(a=17, b=23)
+            give_me_one(a=17, b=set())
         with pytest.raises(AssertionError):
             give_me_one()