Node.js simple script memory leaking

Here is a simple code

var fn = function () {
    return null;
}

setInterval(fn, 100);

setInterval(function () {
    global.gc();
    console.log(process.memoryUsage().rss);
}, 1000);

Runnig

node --expose-gc /tmp/test.js

Result:

10330112
10596352
10596352
10596352
10932224
10932224
10932224
... several minutes 
11198464
11198464
11198464

Why memory is increasing?