R Debianのインストール - エラー

仮想マシンからRをダウンロードしようとしていますが、何らかの理由で次のエラーが発生します。

~$ sudo apt install r-base
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
r-base : Depends: r-base-core (>= 3.6.3-1~bustercran.0) but it is not going to be installed
      Depends: r-recommended (= 3.6.3-1~bustercran.0) but it is not going to be installed
      Recommends: r-base-html but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

実行すると、sudo nano /etc/apt/sources.list以下が表示されます。

deb http://deb.debian.org/debian/ stretch main
deb-src http://deb.debian.org/debian/ stretch main
deb http://security.debian.org/ stretch/updates main
deb-src http://security.debian.org/ stretch/updates main
deb http://deb.debian.org/debian/ stretch-updates main
deb-src http://deb.debian.org/debian/ stretch-updates main
deb [arch=amd64] https://download.docker.com/linux/debian stretch stable
deb-src [arch=amd64] https://download.docker.com/linux/debian stretch stable
deb https://cloud.r-project.org/bin/linux/debian buster-cran35/

実行すると、apt-cache policy r-base以下が表示されます。

r-base:
Installed: (none)
Candidate: 3.6.3-1~bustercran.0
Version table:
 3.6.3-1~bustercran.0 500
    500 https://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages
 3.6.2-1~bustercran.0 500
    500 https://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages
 3.6.1-2~bustercran.0 500
    500 https://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages
 3.6.0-2~bustercran.0 500
    500 https://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages
 3.6.0-1~bustercran.0 500
    500 https://cloud.r-project.org/bin/linux/debian buster-cran35/ Packages
 3.3.3-1 500
    500 http://deb.debian.org/debian stretch/main amd64 Packages

何が問題なのか知っていますか？