Fatal Error – Clues what to do?

When I try to make a post I am getting this error whenever I try to either save a draft or publish:

Fatal error: Call to undefined method SearchSpider::_weak_escape() in /home/nore9152/public_html/home/wp-includes/wp-db.php on line 487

I have no idea what my next moves should be – is this a problem with my WordPress installation or should I be contacting the hosting company? I’ve looked inside the wp-db.php file and from what I can tell line 487 is either empty or contains only ‘*’ (depending whether you count empty lines or not).

Any clues as to what to do would be much appreciated