PNG behave conflict with XML sitemap module

I've noticed that I get a function undefined error in PNG Behave with XML Sitemap 5.x-1.6 module enabled.

Added test in pngbehave.module to ensure function exists to prevent XML sitemap error occuring www.example.com/sitemap.xml

if (function_exists('drupal_set_html_head')) {
    drupal_set_html_head('<!--[if lt IE 7]>
         <script type="text/javascript">
           document.execCommand("BackgroundImageCache", false, true);
           var BlankImgPath = "'. base_path() . drupal_get_path('module', 'pngbehave') .'";
				</script>
        <style type="text/css">' . $pngbehaveclasses . ' { behavior: url('. base_path() . drupal_get_path('module', 'pngbehave') . '/iepngfix.htc)} ' . $pngbehaveexclude . '</style>
        <script type="text/javascript" src="'. base_path() . drupal_get_path('module', 'pngbehave') . '/iepngfix_tilebg.js"></script>
    <![endif]-->');
    }