why trigger not execute??

I testing trigger with pl/pgsql...
i face with some error!!
but, i don't know why error occured??
Please, Would you mind help me?

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CREATE TABLE t1 (a int4, b text, mtime datetime);

CREATE FUNCTION t1_stamp () RETURNS opaque AS '
BEGIN
new.mtime := ''now'';
RETURN new;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER t1_stamp BEFORE INSERT OR UPDATE ON t1
FOR EACH ROW EXECUTE PROCEDURE t1_stamp();
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then " insert into t1 (a) values (1); " running....
this error is occured..
error message is "Erro!! fmgr_info: function 90208: cache lookup failed"