store.select function not working as expected

[Memeplexx]

Minimal reproduction of the bug/regression with instructions:

After upgrading my NGRX store to the latest version, I have been trying to configure my store so that production builds do not fail (because of AOT requirements). I followed this example:
#306 (comment)

Having now done that, I am suddenly having issues selecting state from the store that is more than 1 level deep in the state tree.
For example:
this prints the complete state tree correctly
this.store.select(s => s).subscribe(s => console.log(s));
whereas this prints 'undefined'
this.store.select(s => s.branch1).subscribe(s => console.log(s));

Here is my example code:
https://stackblitz.com/edit/angular-fbmtou

Expected behavior:

I expect to select from the state tree more than 1 level deep

Versions of NgRx, Angular, Node, affected browser(s) and operating system(s):

NGRX: 6.0.1
Angular: 6.0.0
Node: 8.10.0
Chrome
Windows 10

[timdeschryver]

As you can see in the first print, the state looks lik this:

{
  app: {
    branch1: {…},
    branch2: {…},
  }
}

This is because of the getReducer function:

export function getReducers() {
  return {
    app: reducers,
  };
}

Where it should be:

export function getReducers() {
  return reducers;
}

Or you should select the branch as: s=> s.app.branch1