"Object is possibly 'undefined'" after assignment with strictNullChecks

[Asvel]

TypeScript Version: 3.1.0-dev.20180822

Search Terms:
Object is possibly 'undefined'.
strictNullChecks

Code

const objA = { a: 1, b: 2 };
type Key = keyof typeof objA;
const objB = {} as { [index in Key] ? : string[] };
for (const key of Object.keys(objA) as Key[]) {
  objB[key] = ['x'];
  console.log(objB[key][0]);
}

Expected behavior:
No error.

Actual behavior:

error TS2532: Object is possibly 'undefined'.

6   console.log(objB[key][0]);
                ~~~~~~~~~

Playground Link:
https://www.typescriptlang.org/play/index.html#src=const%20objA%20%3D%20%7B%20a%3A%201%2C%20b%3A%202%20%7D%3B%0D%0Atype%20Key%20%3D%20keyof%20typeof%20objA%3B%0D%0Aconst%20objB%20%3D%20%7B%7D%20as%20%7B%20%5Bindex%20in%20Key%5D%20%3F%20%3A%20string%5B%5D%20%7D%3B%0D%0Afor%20(const%20key%20of%20Object.keys(objA)%20as%20Key%5B%5D)%20%7B%0D%0A%20%20objB%5Bkey%5D%20%3D%20%5B'x'%5D%3B%0D%0A%20%20console.log(objB%5Bkey%5D%5B0%5D)%3B%0D%0A%7D

Control flow analysis only works for known keys (like obj.x) and not computed keys. So it won't work for an object key stored in a variable.

[Asvel]

Thanks.

[iwe7]

tsconfig -> strictNullChecks:false