apt get - apt-get wants to remove MariaDB on Debian Wheezy

I've set up my server to use MariaDB and used the installation instructions on the web page. It worked quite well. Now, if I want to dist-upgrade, apt-get wants to replace MariaDB with MySQL:

The following packages will be REMOVED:
  libmariadbclient18 mariadb-client-5.5 mariadb-client-core-5.5 mariadb-server mariadb-server-5.5 mariadb-server-core-5.5
...
The following packages will be upgraded:
  libmysqlclient18 mysql-common

Why does apt-get want to do this? How can I stop apt-get from doing so?