angularfire: app.auth is not a function

Version info

Angular: 6.0.0

Firebase: 5.0.4

AngularFire: 5.0.0-rc.10

Other (e.g. Ionic/Cordova, Node, browser, operating system): WSL, Node 8.11.2, Cordova, etc.

I receive the message:

app.auth is not a function

This occurs in the following code on the line return app.auth():

var AngularFireAuth = (function () {
  function AngularFireAuth(options, nameOrConfig, platformId, zone) {
    var _this = this;
    this.zone = zone;
    var scheduler = new angularfire2__WEBPACK_IMPORTED_MODULE_3__["FirebaseZoneScheduler"](zone, platformId);
    this.auth = zone.runOutsideAngular(function () {
        var app = Object(angularfire2__WEBPACK_IMPORTED_MODULE_3__["_firebaseAppFactory"])(options, nameOrConfig);
        return app.auth();
    });

app seems to be well-formed:

but it has no auth property. I am instantiating (or rather, Angular is instantiating) AngularFireAuth because it is injected into a service, which itself is instantiated quite early in the application life cycle. Was there some change in how soon the app is completely ready, including its auth property? Or is this possibly some kind of version conflict?