create function problem

I want to index my table using the year of the date. So I create the
following function.

create function to_year(date) returns varchar as '
declare
str varchar;
begin
str := to_char($1, "YYYY");
return str;
end;
' language 'plpgsql' with (iscachable, isstrict);

But I face the problem as follow
select to_year(current_date);
WARNING: Error occurred while executing PL/pgSQL functions to_year
WARNING: line 4 at assignment
ERROR: Attribute "YYYY" not found

Anybody can help? Thanks.

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