Could not load assembly 'System.Web'

I am adding a reference in Portable Class Library,while rebuilding it gives error mentioned below

Exception while loading assemblies: System.IO.FileNotFoundException: Could not load assembly 'System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. Perhaps it doesn't exist in the Mono for Android profile?
File name: 'System.Web.dll'
at Xamarin.Android.Tuner.DirectoryAssemblyResolver.Resolve(AssemblyNameReference reference, ReaderParameters parameters)
at Xamarin.Android.Tasks.ResolveAssemblies.AddAssemblyReferences(ICollection

1 assemblies, AssemblyDefinition assembly, Boolean topLevel)
   at Xamarin.Android.Tasks.ResolveAssemblies.AddAssemblyReferences(ICollection