RandomNumberGenerator in ASP.NET5

I have created a new Web API project using the ASP.NET 5 Templates. I am wanting to generate random numbers using System.Security.Cryptography.RandomNumberGenerator but it is not available in the .NET Platform 5.4 (see screenshot).

Compiling it also generates the following errors:

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

Error   CS0246  The type or namespace name 'RandomNumberGenerator' could not be found (are you missing a using directive or an assembly reference?)

Error   CS0103  The name 'RandomNumberGenerator' does not exist in the current context

Is there a way to use this or is there an alternative random generator that I can use which is cryptographically secure?