Warning: SimpleXMLElement::addAttribute(): string is not in UTF-8

Warning: SimpleXMLElement::addAttribute(): string is not in UTF-8

[Symfony\Component\Debug\Exception\ContextErrorException]
Warning: SimpleXMLElement::addAttribute(): string is not in UTF-8 in
W:\public_html\Project\vendor\doctrine\orm\lib\Doctrine\ORM\Tools\Export\Driver\XmlExporter.php
line 162
I have no idea why?