Cyrillic Encoding Issue

[Yasen]

Hello,

When I try to input Bulgarian Cyrillic characters via drawText(), I get this error:
Notice: iconv(): Detected an illegal character in input string in .../vendor/zendframework/zendpdf/library/ZendPdf/Resource/Font/Simple/AbstractSimple.php on line 253

Here's a sample
$page1->setFont($font, $headerSize)->drawText('Основна', 0, 800);

Tried to play with some other options, but no luck. Any chance for fix or advice? Thank you.