CFA not work in function #13142

TypeScript Version: 2.1.1 / nightly (2.2.0-dev.201xxxxx)
2.1.4

Code

let f:string | null = 'xx';
let v:string;
if(f){
    [].forEach(item=>{
        v = f;
    })
}

Expected behavior:
Compile successfully

Actual behavior:

test.ts(5,9): error TS2322: Type 'string | null' is not assignable to type 'string'.
Type 'null' is not assignable to type 'string'.