Similarity function in Postgres with pg_trgm

I'm trying to use the similarity function in Postgres to do some fuzzy text matching, however whenever I try to use it I get the error:

function similarity(character varying, unknown) does not exist

If I add explicit casts to text I get the error:

function similarity(text, text) does not exist

My query is:

SELECT (similarity("table"."field"::text, %s::text)) AS "similarity", "table".* FROM "table" WHERE similarity > .5 ORDER BY "similarity" DESC LIMIT 10

Do I need to do something to initalize pg_trgm?