Typescript: Indexer enforces property compliance

Here is a new thing I found about typescript 0.9Alpha. Once you specify a string indexer all other properties must be of the same type (which makes sense):

interface IBar{
    bar: string;
}

interface IFoo{
    a?: {
        [key: string]: IBar; // Once you have a string indexer all other properties
        options?: IBar;        // e.g Options, must have the same type
    }
}

var x: IFoo = {
    a: {        
        'someKey': {
            bar:'asdf' // put any thing other than a string here and you get an error
        }        
    }
}