diff --git a/tests/unit/test_serialize.py b/tests/unit/test_serialize.py
index f063625..5f2a9aa 100644
--- a/tests/unit/test_serialize.py
+++ b/tests/unit/test_serialize.py
@@ -31,4 +31,5 @@ def test_deserialize_new_json_cassette():
                    side_effect=UnicodeDecodeError('utf-8', b'unicode error in serialization',
                                                   0, 10, 'blew up'))
 def test_serialize_constructs_UnicodeDecodeError(mock_dumps):
-    jsonserializer.serialize({})
+    with pytest.raises(UnicodeDecodeError):
+        jsonserializer.serialize({})
diff --git a/vcr/serializers/jsonserializer.py b/vcr/serializers/jsonserializer.py
index 5b0e2da..b701040 100644
--- a/vcr/serializers/jsonserializer.py
+++ b/vcr/serializers/jsonserializer.py
@@ -17,7 +17,7 @@ def serialize(cassette_dict):
             b"Error serializing cassette to JSON",
             original.start,
             original.end,
-            original.message +
+            original.args[-1] +
             ("Does this HTTP interaction contain binary data? "
              "If so, use a different serializer (like the yaml serializer) "
              "for this request?")