pluggable.php error with www URL

Hi WP,

When I type in URL.com my site works fine, but when I type in https://www.URL.com, I get this error?

Warning: Cannot modify header information – headers already sent by (output started at /home/content/09/8035009/html/index.php:1) in /home/content/09/8035009/html/wp-includes/pluggable.php on line 866

Any thoughts on how to fix it?

Thanks!