strictNullChecks not recognizing evaluated key in object

TypeScript Version: 2.3.2

Code
This version errors with strictNullChecks

export interface Opts {
    params?: { [s: string]: string };
}

let token;
let options: Opts = { params: {} };

if (token) {
    options.params["access_token"] = token;
}

This version does not error with strictNullChecks

export interface Opts {
    params?: { [s: string]: string };
}

let token;
let options: Opts = {};

options.params = {};

if (token) {
    options.params["access_token"] = token;
}

Expected behavior:
TS should recognize that params has been set in the object like it already does using a differently formatted method of initializing the variable.

Actual behavior:
TS fails to recognize that params has been set.

Reproduce both TypeScript 2.3.2 snippets with strictNullChecks enabled and compare their control-flow analysis of the optional params property. Trace the strictNullChecks type-checking entry point that handles property assignments and indexed access. Done means the object-literal initialization form recognizes params as assigned consistently with the separately assigned form.