Inserting into collection MongoDB+node.js

I'm doing tutorial from here: node-mongodb-native
And I don't know why sth like this works:

collection.insert(doc, {w:1}, function(err, result) {});
collection.update({mykey:1}, {$set:{fieldtoupdate:2}}, {w:1}, function(err, result) {});

And this doesn't:

collection.insert(doc, {w:1}, function(err, result) {
   collection.update({mykey:1}, {$set:{fieldtoupdate:2}}, {w:1}, function(err, result) {});
});

Any ideas?