Upgrade problem

While updating to 3.7.1 getting this error

Downloading update from https://www.ads-software.com/wordpress-3.7.1-no-content.zip…

Unpacking the update…

Fatal error: Call to undefined function mbstring_binary_safe_encoding() in /home/content/95/10540895/html/tammathewriter/wp-admin/includes/class-wp-filesystem-direct.php on line 62