Blank SWF in IE6

I have SWF's embeded in my node.tpl.php like so:

<?php
$flashvars = $node->nid;
print swf(
'objectViewer-Test1.swf', '500x400', "flashvars=" . $flashvars
);
?>

Works okay except for in IE6 (haven't tried IE7) where SWFs' come up blank?! What could the issue be here that IE doesn't like?
I'm also using "Embed Flash Directly, don't use JavaScript replacement."