help me, please

I have a problem
I have a function in plpgsql:

create function add_one (integer) returns integer as '
begin
return $1+1;
end;
' language 'plpgsql';

When I create in psql monitor this function the message is CREATED
Then I write:
select add_one(1);
and always say:

fmgr_info: function 18816: cache lookup failed

This happend when I write any function.
How Can I resolve this problem?