apt-get: Why "Unable to find source package" errors?

[Kynn Jones <kynnjo@gmail.com>]

Example:

    % sudo apt-get build-dep emacs25

    Reading package lists... Done
    E: Unable to find a source package for emacs25

I have run `apt-get update` before running the command above, and my `/etc/apt/sources.list` file contains the following

    deb  http://deb.debian.org/debian stretch main contrib non-free
    deb-src  http://deb.debian.org/debian stretch main contrib non-free

    deb  http://deb.debian.org/debian stretch-updates main contrib non-free
    deb-src  http://deb.debian.org/debian stretch-updates main contrib non-free

    deb http://security.debian.org/ stretch/updates main contrib non-free
    deb-src http://security.debian.org/ stretch/updates main contrib non-free

What am I doing wrong?