rmail invalid mbox file

[Jim Erickson]

i am using rmail in emacs 27.2 on openbsd 6.9 on raspberrypi 4B 8gb
aarch64. i use fetchmail and procmail to deliver my gmail to
/home/jim/RMAIL. all rmail settings are at standard defaults. when i
try to open rmail to process my gmail it says the following:

Debugger entered--Lisp error: (error "Invalid mbox file")
  signal(error ("Invalid mbox file"))
  error("Invalid mbox file")
  rmail-convert-file-maybe()
  rmail()
  funcall-interactively(rmail)
  call-interactively(rmail)
  menu-bar-read-mail()
  funcall-interactively(menu-bar-read-mail)
  call-interactively(menu-bar-read-mail nil nil)
  command-execute(menu-bar-read-mail)

i am using the standard RMAIL file formed by receiving mail through
fetchmail + procmail. can anyone tell me how to form a valid mbox file
so i can use rmail to process and read my gmail?