Postgresql: SERIAL incremented on failed constraint INSERT

Having a simple table structure like this:

CREATE TABLE test (
    id INT PRIMARY KEY,
    sid SERIAL
);

I noticed if I attempt to insert a row but it fails a constraint test (i.e. PRIMARY KEY constraint), the SERIAL counter will increment anyway, so the next successful insert, sid will be sid + 2 instead of sid + 1.

Is this normal behavior? Any way to prevent this?