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debootstrap error with dvdrom install

I'm trying to install Stretch on a Lenovo laptop with the KDE dvdrom and each time I get the error:
  debootstrap error. Unable to find the codename for release

I saw a lot of posts about this error, but they were all related to a usb install, but one.
This one said he left the install in standby for several hours, but this is not my case.
So, I don't know what to do. Has anybody an idea?

best regards,
Pierre Frenkiel