Using a parameter in Interval

No matter how I try to concatenate, I can't seem to get a parameter to be used by INTERVAL in a function:

CREATE OR REPLACE FUNCTION Testing(TrailingWeeks int) RETURNS date AS $$
BEGIN
RETURN current_date - INTERVAL (CAST(TrailingWeeks AS varchar) || ' weeks');
END;
$$ LANGUAGE plpgsql;

--select * from testing(1);

ERROR: syntax error at or near "CAST" at character 34
QUERY: SELECT current_date - INTERVAL (CAST( $1 AS varchar) || ' weeks')
CONTEXT: SQL statement in PL/PgSQL function "testing" near line 2

I have tried concatenating it as a declared variable (with and without apostrophes)
1 weeks
And
'1 weeks'

With no success. Any tips?

Many thanks,

Robert