Strange SERIAL / Sequence error

I create a table with:
CREATE TABLE "tab" ("asdf" SERIAL, "asdf2" char (20) NOT NULL );

Then I try to insert some data:
INSERT INTO "tab" ("asdf", "asdf2") VALUES
(NEXTVAL('""tab_asdf_seq""'::text), 'asdf')

ERROR: Relation '"tab_asdf_seq"' does not exist

(done with phpPgAdmin 2.3)