Catchable Error When Posting A Comment

After someone clicks Post Comment on my blog (http://simplykellydesigns.com/blog), this error is shown:

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/seaturtlebean/simplykellydesigns.com/blog/wp-includes/formatting.php on line 766

The part of formatting.php it is referring to is the last line in this code:

/**
 * Checks for invalid UTF8 in a string.
 *
 * @since 2.8.0
 *
 * @param string $string The text which is to be checked.
 * @param boolean $strip Optional. Whether to attempt to strip out invalid UTF8. Default is false.
 * @return string The checked text.
 */
function wp_check_invalid_utf8( $string, $strip = false ) {
	$string = (string) $string;

How do I find out where the error is that is causing this? The last comment I received was on Thursday and I don’t think I have updated anything since then.