System.Security.Cryptography.ProtectedData error when running application

Hi all,
I have a web app that I am working on and am having a problem using the System.Security.Cryptography.ProtectedData class. I have a reference in my project to the System.Security dll. I defined a new class under the App_Code folder and set the Build Action of the class to Compile. This class is responsible for encrypting data using the System.Security.Cryptography.ProtectedData class.
I can successfully compile the project and Intellisense works perfectly. My problem occurs when I go to run the application. When the app goes to run, I immediately get the error:
The type or namespace name 'ProtectedData' does not exist in the namespace 'System.Security.Cryptography' (are you missing an assembly reference?)
If I move the line of code to one of the code-behind files and run, it works fine. I know I must be doing something wrong, but I can't put my finger on it. I am about ready to jump out the window, because I know it's something st ...