FIREBASE WARNING: Invalid query string segment:

Question

I keep having these warnings in the Firebase Logs Console: "FIREBASE WARNING: Invalid query string segment:"

I checked everything but I can't find some reasons.

Do you have any ideas ?

Configuration

{
"name": "functions",
"description": "Cloud Functions for Firebase",
"dependencies": {
"firebase-admin": "~5.8.1",
"firebase-functions": "^0.8.1"
},
"private": true
}

Test case: a very simple cloud function:

exports.test = functions.database
.ref('/scores/{sportId}/{matchId}')
.onWrite(event => {

const sportId = event.params.sportId;
const matchId = event.params.matchId;

const awayTeamScore = event.data.child("awayTeamScore").val();
const dateLastUpdate = event.data.child("dateLastUpdate").val();
const homeTeamScore = event.data.child("homeTeamScore").val();
const status = event.data.child("status").val();

var awayTeamScoreSnapshot = event.data.child("awayTeamScore");
var dateLastUpdateSnapshot = event.data.child("dateLastUpdate");
var homeTeamScoreSnapshot = event.data.child("homeTeamScore");
var statusSnapshot = event.data.child("status");

return admin.database().ref(/indexMatchLeague/${matchId}).once('value').then(league => {
console.log(matchId, homeTeamScore, awayTeamScore, status);
/*
var promises = [];
league.forEach(leagueSnap => {
const leagueId = leagueSnap.key;
const leagueVal = leagueSnap.val();
console.log(leagueId, leagueVal);
});
return Promise.all(promises);
*/
return null;
});
});