Typescript strictNullChecks and arrays

I do not fully understand Typescript's behavior with compiler option strictNullChecks enabled. It seems that sometimes Typescript (version 2.4.1) understands that an item in a string[] is a string, and sometimes it does not:

interface MyMap {
    [key: string]: string[];
}

function f(myMap: MyMap) {
    const keys = Object.keys(myMap); // keys: string[] => Fine.
    for (let key of keys) { // key: string | undefined => Why?
        key = key as string // So a cast is needed.
        const strings = myMap[key]; // strings: string[] => Fine.
        const s = strings[0]; // s: string => Fine.

        // Error:
        // Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
        // Type 'undefined' is not assignable to type 'string'.
        useVarArgs(...strings);
    }
}
function useVarArgs(...strings: string[]) {
}

Update 2017-07-14:

This strange behavior is only observed when using downlevelIteration. My tsconfig.json:

{
  "compilerOptions": {
    "target": "es5",
    "outDir": "target",
    "downlevelIteration": true,
    "strictNullChecks": true
  }
}