Promise return type error after TypeScript upgrade

Using typescript v2.3.4, the following code worked fine:

    getStuff() : Promise<IStuff>{

        return this.http.get(url, options)
            .toPromise()
            .then( (res) => {
                let stuff: IStuff;
                if (res.json().data){
                    stuff = res.json().data;
                }
                return stuff;
            })
            .catch( (reason) => {
                this.handleError(reason);
            });
    }

...where handleError is like this:

handleError = (error:any) => {
    this.debug(error);
    throw error;
}; 

Now, with typescript v2.4.1 I get the error: 'Type 'Promise<void | IStuff>' is not assignable to type 'Promise<IStuff>'. Type 'void | IStuff' is not assignable to type 'IStuff'. Type 'void' is not assignable to type 'IStuff'.'

I can see why that would be the case.

...but this DOES work, and it makes no sense to me that it would work when the other code does not:

getStuff() : Promise<IStuff>{

    return this.http.get(url, options)
        .toPromise()
        .then( (res) => {
            let stuff: IStuff;
            if (res.json().data){
                stuff = res.json().data;
            }
            return stuff;
        })
        .catch( (reason) => {
                if( reason.status) {
                    return Promise.reject(reason);
                } else {
                    this.handleError(reason);
                }
        });
}

...in the else case, it is doing exactly what the code above that generates the error is doing, and yet there's no error.

I can fix the problem by just changing handleError to be:

handleError = (error:any):Promise<any> => {
    this.debug(error);
    Promise.reject(error);
};

...but I'm curious what the specific change was that caused this to become an error, and why does adding the if/else block work correctly when it has an else path that is identical to the original code?