Error in my HTTP > XI > RFC (BAPI) senario

I have everything setup for the scenario, all of the RFC adapters have a green light in the adapter monitor and I run the 'Test Configuration' and everything runs fine. Though, when I send a HTTP request to the scenario I receive the following error

REQUEST: **************\n

POST /sap/xi/adapter_plain?service=Shaun&namespace=http%3A%2F%2FMyCompany%2Ecom%2Fpoc%2Fhttp2bapi%2FHTML&interface=BAPI_MATERIAL_GET_DETAIL HTTP/1.1\r

\n

Host:myserver.com\r\n

Accept: /\r\n

Content-Length:181\r\n

Authorization: Basic c21hbGxvbmU6Ym8kJDQyOSE=\r\n

\r\n

<?xml version="1.0" encoding="UTF-8"?>\r\n

<rfc:BAPI_MATERIAL_GET_DETAIL xmlns:rfc="urn:sap-com:document:sap:rfc:functions">\r\n

<MATERIAL>2051</MATERIAL>\r\n

</rfc:BAPI_MATERIAL_GET_DETAIL>

RESPONSE: **************\n

HTTP/1.1 500 Internal Server Error\r\n

content-type: text/html; charset=utf-8\r\n

content-length: 173\r\n

server: SAP Web Application Server (1.0;640)\r\n

\r\n

<SAP:Error><SAP:Category>XIServer</SAP:Category><SAP:Code>RCVR_DETERMINATION.NO_RECEIVER_CASE_BE</SAP:Code><SAP:Stack>No receiver could be determined</S

AP:Stack></SAP:Error>

finished.