FileNotFoundException when using System.Xml

When I’m simply using the System.Xml, Unity trows this exception:

FileNotFoundException: Could not load file or assembly ‘System.Xml.Serialization, Version=2.0.5.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35’ or one of its dependencies.

Why is this ?

I’ve found out Unity only trows the exception when I a have a System.ServiceModel.Syndication.dll file in my asset folder, which I need to read RSS feed. How come is this?