missing argument list causes protocol.Error exception in crosserbar.io

Hi,

1. the callee registers an RPC function.
2. The caller calls the function.
3. The function throws an exception in the callee's code.
4. The callee responds with ERROR for the INVOCATION, in the error message with no
   Arguments|list but only ArgumentsKw|dict. The following error is thrown by the router,
   instead of forwarding the ERROR to the CALLER:

Message (after JSON serialization):
"[8,68,1,{},\"custom.error\",{\"message\":\"This error should be thrown\"}]"

crossbar.io's Excpetion (in trace level logging):
crossbar-service_1  | [Router         14 crossbar.router.protocol.WampWebSocketServerProtocol] Traceback (most recent call last):
crossbar-service_1  |   File "/usr/local/lib/python3.5/site-packages/autobahn/wamp/websocket.py", line 88, in onMessage
crossbar-service_1  |     for msg in self._serializer.unserialize(payload, isBinary):
crossbar-service_1  |   File "/usr/local/lib/python3.5/site-packages/autobahn/wamp/serializer.py", line 131, in unserialize
crossbar-service_1  |     msg = Klass.parse(raw_msg)
crossbar-service_1  |   File "/usr/local/lib/python3.5/site-packages/autobahn/wamp/message.py", line 1036, in parse
crossbar-service_1  |     raise ProtocolError("invalid type {0} for 'args' in ERROR".format(type(args)))
crossbar-service_1  | autobahn.wamp.exception.ProtocolError: invalid type <class 'dict'> for 'args' in ERROR

This is against in the recommendation of the standrad's chapter 6.7

"Implementations SHOULD avoid sending empty "Arguments" lists.
E.g. a "CALL" message
[CALL, Request|id, Options|dict, Procedure|uri,
Arguments|list]
where "Arguments == []" SHOULD be avoided, and instead
[CALL, Request|id, Options|dict, Procedure|uri]
SHOULD be sent."

If the empty argument list is sent, the router working properly.
"[8,68,1,{},\"custom.error\",[],{\"message\":\"This error should be thrown\"}]"
This version is harmonize with the standard's chapter 9.2.5, however an example for it could be handy.

The exception looks like come from here: https://github.com/crossbario/autobahn-python/blob/master/autobahn/wamp/message.py#L1134
The code is prepared for the position based argument passing through.

It is a bug in crossbar.io, or the standard is outdated, and ok to send empty argument list, if argument dict is provided?

Thanks for the answer.

br,
Daniel

Start with autobahn/wamp/message.py around the referenced line 1134 and reproduce parsing the two serialized ERROR messages shown in the report. Compare the parser behavior with WAMP specification chapters 6.7 and 9.2.5; done means the expected argument-list and argument-dictionary form is established and covered by a regression test or the issue is confirmed as a crossbar.io problem.