PHPWord/templateProcessor opens the document, replaces the placeholder in the template but saves an empty document

I downloaded the PHPWord library and so far I am quite happy with the results I guess - unless that I can't save a document properly - it does save the document but its empty.

This is my code:

        preg_match_all("/\.(?!.*\.)(.+)/", $word_document, $mime); 
        $mime = $mime[0][0]; // contains ".docx" now

        $templateProcessor = new \PhpOffice\PhpWord\TemplateProcessor($word_document); 
        // $word_document contains the absolute path to the doc
        $templateProcessor->setValue('PLACEHOLDER', 'Doe'); 
        // ${PLACEHOLDER} in doc is replaced with "Doe"

        var_dump($templateProcessor); 
        /* returns, among others, ["tempDocumentMainPart":protected]=>
        string(836) "Behördenvollmacht für Doe" -> you can see that 'Doe' is now contained in the 
        document. */

        $templateProcessor->saveAs($path.$new_doc_name.$mime); // saves the document but its empty

Any tips or ideas why this is not working properly?