New Post: Error: encoding iso-8859-1 not supported

My code is exactly the same I use in ASP:NET applications, but in Silverlight apps I get the error.
Tweetinvi API version: 0.9.8.2 installed by Nuget on Visual Studio 2013.
In asp.net I send twits programmatically with text and image. In Silverlight I get the error on the Publish statement.
My PC is in Italian Windows version.
The error says:" Encoding name 'iso-8859-' not supported. parameter name."

• If a send the exactly same message with the same image manually on the web I have no problems.
  

The code I use is exactly the same that works fine in ASP.Net applications,.

Any suggestions? What does it mean the error message?