Why operationContext.IncomingMessageHeaders.Action is null in DotNetOpenAuth

Question

I try to create a wcf service that supports OAuth.

I started with this example http://www.theleagueofpaul.com/blog/2010/05/11/ooo-aka-openid-odata-and-oauth-together/ but some modification but now I have problems with operationContext.IncomingMessageHeaders.Action because is null.

if (scopes.Contains(operationContext.IncomingMessageHeaders.Action))
{
    return true;
}

Do you have any idea why this is null?