Can't add Shared Project to References in Windows Phone 8.1

Hello,

I'm trying to add a Windows Phone 8.1 project to my existing solution which contains a Shared Project. I'm also using Xamarin.Forms.

But when I follow these steps, I get stuck at the step where I need to add the Shared Project to the References of the Windows Phone 8.1 project. However, when I click Add Reference, I can't find the Shared Project in my list of projects. The Android project does show up though. See screenshot below.

How can I add the Shared Project now? Thanks in advance!