I was having the same trouble and found the following forum post solved the problem. I didn’t want to have to edit the core WordPress files but I couldn’t find any other solution. One difference to note is that in v2.6.2 the line is 295 in wp-includes/classes.php
http://ww.wp.xz.cn/support/topic/33805?replies=8#post-378560
Many thanks to the original poster of the solution, hope this link helps other people out too as long as you don’t mind making the change each time you upgrade.
