Unity Internal Compiler Error, not there in Editor

Hi, I'm using Unity with Visual Studio 2012 as my editor. I am referencing System.Web and using System.Web.HttpUtility for encoding a url. There are no errors showing up in Visual Studio when I build now, but Unity is still not letting me run the script. I'm getting the message below in the console, and when I try to run it it tells me all compiler errors must be solved. But what is the error?? o.O

Assets/Scripts/FormElementCollection.cs(31,43): error CS0234: The type or namespace name HttpUtility' does not exist in the namespace System.Web'. Are you missing an assembly reference?

Don't know if any code would help here, but let me know if there is anything that might help.

Cheers!