"Body schema is missing" error for generating RSP (BTAHL7 2.5.1) message

Hello, I am working on a WCF - web service where service accepts QBP message (HL7 string) as a request and sends RSP message (HL7 string) response back. Through single orchestration, biztalk process QBP message,call a receive pipeline disassembler and then pass query parameters from QBP message to database and retrieve patient etc details. Now I need to construct RSP (response message) back based on database results. I am able to load database fields into RSP multipart-message but when I try to call Send Pipleline, I keep getting Body Schema Missing error. The send pipeline suppose to convert RSP Multipart mesage and return back HL7 message string. Below is the code that I am using to call Send Pipeline.

msgRAWString=new Helper.BizTalkLibrary.RawString();

//RSPBodySegments and RSPMSHSegment are two message variables which are constructed by transformation in earlier steps.

msgRSP.BodySegments    =   RSPBodySegments;         //RSP_K11_251_GLO_DEF -- namespace ishttp://microsoft.com/HealthCare/HL7/2X
msgRSP.MSHSegment    =   RSPMSHSegment;              //MSH_25_GLO_DEF -- namespace ishttp://microsoft.com/HealthCare/HL7/2X

msgRSP.ZSegments = new System.Xml.XmlDocument();

// Call Send Pipeline
SendPipelineInputMessages.Add(msgRSP); 

Microsoft.XLANGs.Pipeline.XLANGPipelineManager.ExecuteSendPipeline(typeof(HL7TargetRSPSendPipeline), SendPipelineInputMessages, msgRAWString );

The above code suppose to return msgRAWString which is an HL7 message.

ANy thought on what I am doing wrong?

THanks!

---Note: I have tested pipeline using this scenario --> Pass the same incoming QBP message (or RSP message), run through HL7 DASM, load it into multipart message variable from DASM and send through SendPipeline, I get HL7 message string as output after calling SendPipeline. The issue I have is when I try to construct multipart message and call Send Pipeline, it throws Body Schema missing error.