Ackerman function causes stack overflow in Red (and not Rebol)

ack.red

Red []
; https://rosettacode.org/wiki/Ackermann_function
ack: func [m n] [
  case [
    0 = m [n + 1]
    0 = n [ack m - 1 1]
    true  [ack m - 1 ack m n - 1]
  ]
]
;
repeat m 3 [
  repeat n 6 [
    print ack m n
  ]
]