indexing large "text" attributes ... ERROR: maximum size is 8191

I get the following error when tying to index a large text field:

dev=# CREATE INDEX idx_fulltext_articel_block ON fulltext_article
(article_block);
ERROR:  index row requires 8724 bytes, maximum size is 8191

I am actually creating a GIN index on another field but I need to
index the original "text" field to perform exact phrase matches.
Does anybody know how to fix this or alternatively perform exact
phrase matches on a GIN index?

Thanks in advance!

Benjamin