'ob_gzhandler' cannot be used after 'URL-Rewriter' ???

[AshleyQuick]

What the heck does this mean and how can I fix it? This is only displayed once per browser session. It disappears by clicking refresh or going back to the page.

Warning: ob_start(): output handler 'ob_gzhandler' cannot be used after 'URL-Rewriter' in /home/public_html/forums/pageheader.php on line 18

Here's the snippet of code for that section:

$gzip_compressed = 0;
if($config['use_gzip_compression'] == 1) {
	if(phpversion() >= "4.0.4") {
		if(extension_loaded("zlib")) {
			ob_start("ob_gzhandler");
			$gzip_compressed = 1;
		}
	}
}

if($gzip_compressed == 0 && $config['activate_ob'] == 1) {
	ob_start();
}