PHP CSSTidy throwing warning unexpectedly

Question

So I have a CSS file which I am running through PHP CSSTidy with the following setup.

require_once './application/helpers/csstidy/class.csstidy.php';

$css = new csstidy();
$css->set_cfg('remove_last_;',TRUE);
$css->set_cfg('compress_colors',TRUE);
$css->set_cfg('template', 'highest');

$css->parse(strip_tags($content));
$content = $css->print->plain();

It seems to work, but I get a notice twice like so:

Severity: Notice  --> Uninitialized string offset: -1 /application/helpers/csstidy/class.csstidy.php 1035
Severity: Notice  --> Uninitialized string offset: -1 /application/helpers/csstidy/class.csstidy.php 1035

Which is this line.

I have no idea why this is happening, the CSS is valid... any ideas? I don't want to mute the notice/warning unnecessarily.

Any ideas? Am I missing something?

In case you missed it this is the link to the CSS file.