My True Black seems to display as Rich Black to the printer.
Hey! So I am mostly a digital artist who rarely has to deal with printing but one of the rare occurences came up and I am really struggling with understanding what could possibly be the issue.
I am working on a childrens book and I have recently sent my files to print. I followed instructions, one of them being that all my blacks should be True Blacks. So I made sure about that - but the printer keeps sending me the files back, claiming that I used Rich Black on the type. Here is the screenshot they send me:

However, I open the same file in my Illustrator and it just seems correct to me. 
Therefore I have no idea how to fix it. Any idea where the problem could be?
Thank you for any suggestions, I have been googling all day and no luck...