Why doesn't String class implement IEnumerable<char> in portable library?

I've created a PCL project that targets .NET Framework 4 and Silverlight 5 (I use an extension for Visual Studio 2010, not portable project template from Visual Studio 2012, if that matters). I am trying to reverse a string using System.Linq.Enumerable.Reverse<TSource>() extension method, but it doesn't work because compiler thinks that System.String doesn't implement IEnumerable<char>.