"Invalid doctype format" when executing Send-pipeline inside an orchestration

Hi

I’m trying to aggregate four messages into one message by providing a list of document messages (variable:objNewListOfDocuments, type:Microsoft.XLANGs.Pipeline.SendPipelineInputMessages (just an assembler component with envelope and document schema specified)) into a send pipeline (GB.Integration.AggregateReading). I know that messages inside this list are correct (I can see it in my log). The aggregated message (variable:msgAggregatedGetMeterReadings, type:System.Xml.XmlDocument) is never created and following error is thrown:

Inner exception: There was a failure executing pipeline “GB.Integration.AggregateReadings”.

Error details: “Invalid doctype format: http://iec.ch/TC57/2011/GetMeterReadings##GetMeterReadings”.
       
Exception type: XLANGPipelineManagerException

Code:

msgAggregatedGetMeterReadings = null;
Microsoft.XLANGs.Pipeline.XLANGPipelineManager.ExecuteSendPipeline(typeof(GB.Integration.AggregateReading),
objNewListOfDocuments, msgAggregatedGetMeterReadings);

The worst part is that error is complaining about http://iec.ch/TC57/2011/GetMeterReadings##GetMeterReadings which I am expecting. I have an envelope and a document schema and I’m able to split a bigger message into several smaller ones (receive pipeline) so I know that those schemas are correct. I can not find keywords “Invalid doctype format” and BizTalk on Google so I’m truly lost.

Any help is highly appreciated?

Kind regards

A.H.