mysql_affected_rows – Server Could Not Be Established

Hey,

Every now and then I get this error at the bottom of my blog:

Warning: mysql_affected_rows(): A link to the server could not be established in /wp-includes/wp-db.php on line 155

So that code is:

if ( preg_match("/^\s*(insert|delete|update|replace) /i",$query) ) {
$this->rows_affected = mysql_affected_rows();

So is this a bug or what and how can I fix it? Everything else seems to work fine that is connecting to the same mysql database ATM.

Thanks for any help.

Will