wit.ai : sessionid undefined in my runActions function - node.js

Question

I am writing my first apps with wit.ai using a node.js backend. I found some posts here similar to my question, but not really the answer :
I use a socket.io to communicate with my node script.
The two relevant parts of my node are :
io.sockets.on('connection', function (socket) {
socket.on('message',
function(data) {
var json = JSON.parse(data);
var sid = json['sessionid'];
console.log("Received on sid " + sid);
if (_sessions[sid] == undefined) {
_sessions[sid] = {};
_sessions[sid].context = {};
}
_sessions[sid].socket = socket;
client.runActions(sid, json['text'], _sessions[sid].context, 30)
.then((context) => {
_sessions[sid].context = context;
}
)
.catch((err) =>
{
console.error('Oops! Got an error from Wit: ', err.stack || err);
}
);
}
);
}
);
========
const actions = {
send(request, response) {
const {sessionId, context, entities} = request;
const {text, quickreplies} = response;
return new Promise(function(resolve, reject) {
var session = _sessions[sessionId];
console.log("-------------------------------");
console.dir(context);
console.log("-------------------------------");
session.socket.emit("message", JSON.stringify(response));
return resolve();
});
},
gettaxi ({sessionid, context, text, entities}) {
return new Promise(function(resolve, reject) {
console.log(`Session ${sessionid} received ${text}`);
var quand = firstEntityValue(entities, "quand");
if (!quand && context['quand'] != undefined) quand = context['quand'];
var depart = firstEntityValue(entities, "depart");
var arrivee = firstEntityValue(entities, "arrivee");
if (depart) {
console.log("Found depart");
context.depart = depart;
delete context.missing_depart;
}
else {
context.missing_depart = true;
}
if (arrivee) {
console.log("Found arrivee");
context.arrivee = arrivee;
delete context.missing_arrivee;
}
else {
context.missing_arrivee = true;
}
console.dir(context);
if (quand) {
console.log("Found quand");
context.quand = quand;
delete context.missing_quand;
}
else {
context.missing_quand = true;
}
return resolve(context);
}
);
},
};
All is working rather good, except than my gettaxi receives a undefined sessionid.
It's not the case for the send function that receives the correct sessionid.
What am I doing wrong ?