currval() Issue in PG

gistfile1.sql

-- A table, "turtles":
CREATE TABLE turtles
(
  id serial NOT NULL,
  name text,
  lastid integer,
  CONSTRAINT pk_turtles PRIMARY KEY (id)
)
WITH (
  OIDS=FALSE
);
ALTER TABLE turtles
  OWNER TO john;
  
-- INSERT, say, 2103 records. 
-- then take curval:
SELECT currval('turtles_id_seq') as newId;
-- returns 2103. Then clear the table:
DELETE FROM turtles

-- Close the session. Come back tomorrow, try curval:
SELECT currval('turtles_id_seq') as newId;

-- Get this:
ERROR: currval of sequence "turtles_id_seq" is not yet defined in this session
SQL state: 55000