Can't install MongoDB for PHP 7.2

I'm trying to use sudo apt-get install php7.2-mongodb on an ubuntu 16.04 droplet running nginx and PHP 7.2, and every time I try to run this line I get:

dpkg: error processing package php7.0-fpm (--configure):
subprocess installed post-installation script returned error exit 
status 1
Errors were encountered while processing:
php7.0-fpm

..and plenty more above that although I'm not sure why it's still trying to use PHP 7.0 here.

I've found walkthroughs for installing mongo and then upgrading PHP from 7.0 to 7.2, but how can I install Mongo after the 7.2 upgrade?

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