simple problem

Hello!
I am having problems creating a table and i'd like some help.

-->create table loco( id_loco int4 not null, primary key (id_loco), check(
(select count(*) from loco ) < 3 ) );

I guess the statment is right, but when:
->insert into loco values(1);

the result is: ERROR:  ExecEvalExpr: unknown expression type 108

How can I fix this problem?

Thanks.