"No cores were discovered" with nrf52840

I have tried stlink and a cmsis-dap dongle, the results are same.

pyocd commander -v --target nrf52840
0000585:INFO:board:Target type is nrf52840
0000610:INFO:dap:DP IDR = 0x2ba01477 (v1 rev2)
0000628:INFO:ap:AP#0 IDR = 0x23000000 (proprietary)
0000631:INFO:ap:AP#1 IDR = 0x02880000 (proprietary)
Exception while initing board: No cores were discovered!
Traceback (most recent call last):
File "c:\python27\lib\site-packages\pyocd\tools\pyocd.py", line 704, in connect
self.session.open(init_board=not self.args.no_init)
File "c:\python27\lib\site-packages\pyocd\core\session.py", line 367, in open
self._board.init()
File "c:\python27\lib\site-packages\pyocd\board\board.py", line 83, in init
self.target.init()
File "c:\python27\lib\site-packages\pyocd\core\coresight_target.py", line 162, in init
seq.invoke()
File "c:\python27\lib\site-packages\pyocd\utility\sequencer.py", line 201, in invoke
resultSequence = call()
File "c:\python27\lib\site-packages\pyocd\core\coresight_target.py", line 242, in check_for_cores
raise exceptions.DebugError("No cores were discovered!")
DebugError: No cores were discovered!

Reproduce the report with pyocd commander -v --target nrf52840 and start at pyocd/core/coresight_target.py, especially check_for_cores, using the traceback as the entry point. Trace the reported AP discovery through board initialization and establish the cause of the missing core; done means the nrf52840 connection succeeds or the failure is clearly diagnosed and documented.