Fatal error: Cannot use string offset as an array in .../includes/theme.inc on line 552

I get this error when clicking on links to nodes or comments, but only when logged in as an Administrator.

I'm using the Zen theme.

Lines 546-554, with the offending line bolded:

// Automatically add a class to each link and also to each LI
if (isset($link['attributes']) && isset($link['attributes']['class'])) {
$link['attributes']['class'] .= ' ' . $key;
$class = $key;
}
else {
$link['attributes']['class'] = $key;
$class = $key;
}