[Budfudder]

I've been using ebook-meta to change the metadata on my .mobi files, and I'm trying to empty as many of the metadata slots as I can. So far I've discovered the following parameters to empty slots:

--tags=""
--publisher=" "
--comments=" "

My first question is why does --publisher=" " empty the publisher slot, rather than --publisher=""? To empty the tags slot you use --tags="" - why not the same for the publisher and comments slots?

My second question is around the Book Producer slot. I can't empty this - I can't even change its content, no matter what I do. I've tried --book-producer="", --book-producer=" " and even --book-producer="test", but it remains the same no matter what. Here's sample output:

X:\Personal\eBooks>ebook-meta esop.mobi --book-producer=""
Original metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

Changed metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

X:\Personal\eBooks>ebook-meta esop.mobi --book-producer=" "
Original metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

Changed metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

X:\Personal\eBooks>ebook-meta esop.mobi --book-producer="test"
Original metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

Changed metadata::
        Title               : Aesop's Fables
        Author(s)           : Aesop
        Book Producer       : calibre (0.7.16) [http://calibre-ebook.com]
        Tags                :
        Published           : 0101-01-01T00:00:00+00:00
        Identifiers         : isbn:9781593080624
        Comments            :

I've tried this with an old version of Calibre (0.8.something), 0.9.5 and the latest, 0.9.22. It seems to not work on any of them. Or am I doing something wrong?

Thanks for any help.