Illegal invocation with document.querySelector

Possible Duplicate:
JavaScript function aliasing doesn't seem to work

Related jsfiddle: http://jsfiddle.net/cWCZs/1/

The following code works perfectly:

var qs = function( s ) {
    return document.querySelector( s );
};
qs( 'some selector' );

But the following doesn't:

var qs = document.querySelector;
qs( 'some selector' ); // Uncaught TypeError: Illegal invocation

I don't understand why.

My confusion comes with the fact that this works:

function t() {
    console.log( 'hi' );
}
var s = t;
s(); // "hi"