posgres optimizer not using the index on hstore HELP

I have a simple query select on the hstore attribute "EMAIL_ADDRESS". but the
thing is i'm using a parameter in the query to specify which hstore
attribute I want to access, and when using a parameter the index which I
created is suppressed; whereas when I hard code the attribute
"EMAIL_ADDRESS" then the index is used. this is the index I created:

create index indx_email on contact using btree
((contact_attributes->'EMAIL_ADDRESS'::text));


the query is:

select * from contact where contact_attributes->$parameter =
"fqwefd@hotmail.com';


when I hardcode the attribute like below it the index is used.

select * from contact where contact_attributes->'EMAIL_ADDRESS' =
"fqwefd@hotmail.com';



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