How can I get the route header within the AllHeaders-Collection in a Managed SIP Application

I try to get the Route Header of a SIP message in the Dispatch-method of a Managed SIP Application.

The OCS Logger shows clearly a route header with a valid content. But there is no header of type 'route' in the eventArgs.Request.AllHeaders object in my Managed SIP Application. The enum 'Header.StandardHeaderType.Route' is defined in 'Microsoft.Rtc.Sip' namespace, so I guess there should be such a route header ?!

How can I get the route header in the Managed SIP Application?