Node.js. Why does this pipe throw: Error: write after end?

Question

I wrote the next simply code:

gulp.task('test', function () {
    var throughStream = through.obj(function (data, enc, next) {
        this.push(data);
        next();
    });

    function processStr(str, cb, parentStream) {
        var stream = new Stream.Readable();
        stream.push(str);
        stream.push(null);

        stream.pipe(throughStream);
        stream.on('data', function (data) {
        });
        stream.once('end', function () {
            cb();
        });
    }


    var stream = new Stream.Readable();
    stream.push("str 1 ");
    stream.push("str 2 ");
    stream.push("str 3 ");
    stream.push(null);

    stream.pipe(through.obj(function (data, enc, next) {
        processStr(data, next, this);
    }));
});

Line: stream.pipe(throughStream); throws error "write after end".

But if I move var throughStream into processStr function then all is okey.

Or if I delete stream.once('end', ...) then it also works good.

So, I can't understand why does it happen?