BTS Project build error on string message

Today Is was making an orchestration that contains a Multi-Part message type, with a message part of type System.String. When building the BizTalk Project it returned the error "'System.String' must be Xml serializable to be a message part type". I've tried several things, but it kept giving this same error no matter what I tried. But I was certain I had used System.String parts in an orchestration before, without any problems. So after some debugging, I tried restarting Visual Studio... and this seemed to do the trick! Apparantly all ik took to succesfully build the biztalk project was restarting Visual Studio.