Android.Net Unkown Resolve Error

I'm trying to open a webpage in a Xamarin.Android app using:

...
using Android.Net;
...

Intent browserIntent = new Intent(Intent.ActionView, new Android.Net.Uri("http://xamarin.com"));
StartActivity(browserIntent);

Because I'm also using System.Uri in the same class I have used the full namespace for Android.Net.Uri.

When I build I get:

Error CS0234: The type or namespace name 'Net' does not exist in the namespace '...' 
(are you missing an assembly reference?) (CS0234)

What am I missing here?

Version 4.8.1 (Business edition)