Invalid JPG-Image in PreviewImage-Tag

The embedded preview in the “PreviewImage” Exif-Tag is not valid for DNG-Files created in PureRAW.
When extracting the preview with exiftool like "exiftool -PreviewImage -b -w ‘%f.jpg’ test.dng there is an Error:
Warning: [minor] PreviewImage is not a valid JPEG image

Can you please correct that and write a valid JPEG preview? Thank you.