Agency administrative subpoena that does not identify the activity under investigation or the relevant provision of law does not allow for judicial review of the reasonableness of the request, making it statutorily infirm.
http://www.ca5.uscourts.gov/opinions/pub/17/17-10732-CV0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.