exiv2 can't read or write to a DNG

Question

I converted a raw Sony file (.ARW) to a .DNG format with Lightroom.

Now I would like to write new keywords to the IPTC of this .dng file with exiv2.

The command exiv2 -P I test5.dng gives me the error message:

"Error: Directory Sony2 with 25665 entries considered invalid; not read."

What can I do? If exiv2 won't be able to do the job, any other software that might work?