Cannot modify header information – headers already sent

I get the following error (Cannot modify header information – headers already sent by (…/functions.php:1)) even when functions.php is EMPTY. I am running a custom wordpress install (I made the theme and am righting all the functions myself). There is no space before or after the <?php ?> tags, and, as I said, this error will occur even when I have nothing in my ‘functions.php’ file. How can I solve this problem.