CultureAndRegionInfoBuilder doesn't exist

Okay, this is a strange one.

I'm trying to create a custom culture using:

using System.Globalization;

...

var x = new CultureAndRegionInfoBuilder();

But I get the nasty red Resharper error with no options.

The type or namespace name 'CultureAndRegionInfoBuilder' does not exist in the namespace 'System.Globalization'

What's going on, am I missing something? Can't find any help on google...