TCPDF ERROR: Could not include font definition file: helvetica

Using TCPDF (latest 6.0.004) as PDF generator.
Maybe it's not module bug, pdf's are generating normally.
Error shows up on admin/config page.
TCPDF ERROR: Could not include font definition file: helvetica

Function AddFont() (tcpdf.php line 4195) produces an error message:
Variable $fontfile contains wrong path to font file, because of empty return value _getfontpath().

include/tcpdf_fonts.php line 1669:
public static function _getfontpath() {
if (!defined('K_PATH_FONTS') AND is_dir(dirname(__FILE__).'/fonts')) {
define('K_PATH_FONTS', dirname(__FILE__).'/fonts/');
}
return defined('K_PATH_FONTS') ? K_PATH_FONTS : '';
}

dirname(__FILE__) is 'include' (include/tcpdf_fonts.php), but path 'include/fonts' is not existing.
K_PATH_FONTS can't be defined, so function returns ''. Wrong fonts directory.

I've copied fonts directory to includes, it's working now. I can't find a reason why tcpdf invoked on administration page.