EmptyPartException

I did not find very much useful information online about this. It happened because of the following code snippet in an expression shape:

Message1 = new System.Xml.XmlDocument();

Message1 was defined with a specific type, but its .NET type is XmlDocument. So I was allowed to assign the new XmlDocument to the message, but sometime after the assignment BizTalk wised up and realized that the contents of the message were empty (and thus did not match the message's schema).