I got this piple line validator error.
The reason causes this problem is the application sends in an empty tag for the field which type is expected as 'Long' by BizTalk schema. By sending an empty tag, BizTalk piple line validator intepreted it as a string type with empty value, therefore failed in type validation.
The point is clear, if you want to validate the message, make sure you don't send in empty tag for any un-string type fields.