AT TIME ZONE INTERVAL and prepared statements

AT TIME ZONE INTERVAL $1 doesn't seem to be supported in prepared
statements.

This works:

SELECT now() AT TIME ZONE INTERVAL '-08:00';

This doesn't:

PREPARE mys(text) AS SELECT now() AT TIME ZONE INTERVAL $1;
ERROR: syntax error at or near "$1" at character 57

These work:

PREPARE mys(interval) AS SELECT now() AT TIME ZONE $1;
PREPARE mys(text) AS SELECT now() AT TIME ZONE ($1::interval)

Any reason why the second case doesn't work?

Kris Jurka