Detected an illegal character in input string iconv()

Hi, I got this PHP Notice:

PHP Notice:  iconv(): Detected an illegal character in input string in wp-content\plugins\woocommerce\includes\wc-core-functions.php on line 1627
PHP Notice:  iconv(): Detected an illegal character in input string in wp-content\plugins\woocommerce\includes\wc-core-functions.php on line 1628

I must fix this by changing:

$a = iconv( 'UTF-8', 'UTF-8//TRANSLIT', $a );
$b = iconv( 'UTF-8', 'UTF-8//TRANSLIT', $b );