Restlet C# httpClient header issue

I am having trouble setting the "Content-Type" header in a httpClient object in C#

http://msdn.microsoft.com/en-us/library/system.net.http.httpclient.aspx

When I do

httpClient.DefaultRequestHeaders.Add("ContentType", "text/plain");

The program will run in Windows, but Netsuite throws me an Error 500

When I do:

httpClient.DefaultRequestHeaders.Add("Content-Type", "text/plain");

I get an error in Visual Studio stating that "Content-Type" is an invalid name.

Any ideas?

Thank you in advance.

**Edit**

The actual error is:

An exception of the type 'System.InvalidOperationException' occurred in System.Net.Http.dll but was not handled in user code