No default importer for DLL when building project after adding reference to Content Pipeline

I’m trying to use Monogame.Extended following the tutorial, but I can’t add the reference to the DLL to the content pipeline.

I locate the DLL and add it through the GUI, but when I try to build the project (even the default that the Monogame templates create for you), it fails with an error that says “there is no default importer for [path to the dll file].dll”, anybody experiencing the same thing?

Using the last version available of both Monogame & Monogame.Extended (& Monogame.Extended.Content.Pipeline).