Error when enable 'merge css files' in magento

[R11baka]

Magento 1.7.0.2
Notice: Uninitialized string offset: 0 in /var/www/vhosts/boxshopdev.dev/lib/Diglin/Csstidy/Print.php on line 170
#0 /var/www/vhosts/boxshopdev.dev/lib/Diglin/Csstidy/Print.php(170): mageCoreErrorHandler(8, 'Uninitialized s...', '/var/www/vhosts...', 170, Array)
#1 /var/www/vhosts/boxshopdev.dev/lib/Diglin/Csstidy/Print.php(98): Diglin_Csstidy_Print->_print(true)
#2 /var/www/vhosts/boxshopdev.dev/app/code/community/Diglin/UIOptimization/Block/Optimize/Head.php(161): Diglin_Csstidy_Print->plain()
#3 /var/www/vhosts/boxshopdev.dev/app/code/community/Diglin/UIOptimization/Block/Optimize/Head.php(59): Diglin_UIOptimization_Block_Optimize_Head->_prepareCssCompression('css/styles.css', Array)
#4 /var/www/vhosts/boxshopdev.dev/app/code/core/Mage/Page/Block/Html/Head.php(202): Diglin_UIOptimization_Block_Optimize_Head->_prepareStaticAndSkinElements('