Postgresql prepared transactions

Hi,

I do following commands

test=# BEGIN;
BEGIN
test=# insert into testxa1 values(1);
INSERT 0 1
test=# insert ddddduuuuuppppp;
ERROR:  syntax error at or near "ddddduuuuuppppp"
LINE 1: insert ddddduuuuuppppp;
               ^
test=# PREPARE TRANSACTION 'a';
ROLLBACK

Why on prepare transaction I got rollback. If I skip error, each RM will try
to do two-phase commit, but it's looks impossible to do two-phase commit.

Regards,
Radek