bug on index of tsearch2?

hi all       i create table and index following the doc of tsearch2-guide.html 
this way:
=#CREATE TABLE docs ( id SERIAL, doc TEXT, vector tsvector )
=# CREATE INDEX docs_index ON docs USING gist(vector);

CREATE FUNCTION insdoc(text) RETURNS void LANGUAGE sql AS 'INSERT INTO docs (doc, vector) VALUES ($1,
to_tsvector($1));'

=# SELECT insdoc('A low crawl over cobbles leads inward to the west.')
=# SELECT insdoc('The canyon runs into a mass of boulders -- dead end.')
=# SELECT insdoc('You are crawling over cobbles in a low passage.')
=# SELECT insdoc('Cavernous passages lead east, north, and south.')
when i debug ,i can break at function such as gistgettuple,gistfirst,gistnext

BUT when i create table and index this way( just create the index after i have inserted data ) ,i can't debug
gist,why?

=#CREATE TABLE docs ( id SERIAL, doc TEXT, vector tsvector )

CREATE FUNCTION insdoc(text) RETURNS void LANGUAGE sql AS 'INSERT INTO docs (doc, vector) VALUES ($1,
to_tsvector($1));'

=# SELECT insdoc('A low crawl over cobbles leads inward to the west.')
=# SELECT insdoc('The canyon runs into a mass of boulders -- dead end.')
=# SELECT insdoc('You are crawling over cobbles in a low passage.')
=# SELECT insdoc('Cavernous passages lead east, north, and south.')

=# CREATE INDEX docs_index ON docs USING gist(vector);