I ran into this while experimenting with alternate page sizes:

  # mkswap -f --label swap -p 2048 swapfile 65536
  mkswap: Using user-specified page size 0, instead of the system value 4096
  mkswap: swapfile: warning: wiping old swap signature.
  Setting up swapspace version 1, size = 64 MiB (67106816 bytes)
  LABEL=swap, UUID=d0521a3a-8594-4d4b-a79b-5259aeac8d45

Note "user-specified page size 0" in the above output.  I've attached
a trival patch to fix that warning message.