Message.INBOUND_MESSAGE in PREPARE_SEND_ENDING Interceptor

I have a CXF service, which is also a WS client of other services

I have a PREPARE_SEND_ENDING Interceptor configured in my service
(which is invoked both when sending a request as a client,
and when returning a response as a server;
so it is invoked twice)

in the interceptor I would like to figure out
which of these two invocations is the current invocation

I thought I could use the Message's INBOUND_MESSAGE attribute
(which is a Boolean);
to my surprise, it returns NULL, when returning the response as server
(and returns false, when sending the request as a client)

isn't it a bug in CXF, that the Message's INBOUND_MESSAGE attribute is NULL?
or am I missing something?