diff --git a/autobahn/wamp/protocol.py b/autobahn/wamp/protocol.py
index 2035d0c1c..ca055c433 100644
--- a/autobahn/wamp/protocol.py
+++ b/autobahn/wamp/protocol.py
@@ -502,13 +502,17 @@ async def _type_check(*args, **kwargs):
             response = []
             for name, kind in func.__annotations__.items():
                 if name in arguments:
+                    # if a "type" has __origin__ attribute it is a
+                    # parameterized generic
                     if getattr(kind, "__origin__", None) == Union:
+                        expected_types = [arg.__name__ for arg in kind.__args__]
                         if not isinstance(arguments[name], kind.__args__):
                             response.append(
-                                "'{}' required={} got={}".format(name, kind.__name__, type(arguments[name]).__name__))
+                                "'{}' expected types={} got={}".format(name, expected_types,
+                                                                       type(arguments[name]).__name__))
                     elif not isinstance(arguments[name], kind):
                         response.append(
-                            "'{}' required={} got={}".format(name, kind.__name__, type(arguments[name]).__name__))
+                            "'{}' expected type={} got={}".format(name, kind.__name__, type(arguments[name]).__name__))
             if response:
                 raise TypeCheckError(', '.join(response))
             return await txaio.as_future(func, *args, **kwargs)