"Call to Hibernate failed: Not enough swap space for hibernation" despite swap file being bigger than RAM

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I have a server running NixOS that I want to enable hibernation on. When I try and run hibernate, I get this error:

[tombert@nixos:~]$ sudo systemctl hibernate
Call to Hibernate failed: Not enough swap space for hibernation

For perspective, I have my swap file set up at /swapfile:

[tombert@nixos:~]$ sudo swapon -s
Filename				Type		Size		Used		Priority
/swapfile                               file		153599996	0		-2

[tombert@nixos:~]$ free -m
               total        used        free      shared  buff/cache   available
Mem:          104720        2148       85366          20       17206      101280
Swap:         149999           0      149999

The swapfile is set here:

swapDevices = [
  { device = "/swapfile";
    size = 150000;

  }
];

I have the kernel boot stuff set here:

boot.resumeDevice = "/dev/disk/by-uuid/45bdbcc1-13d2-417f-a763-ce60c73e42e4";
boot.kernelParams =
[ "processor.max_cstate=1"
  "resume=UUID=45bdbcc1-13d2-417f-a763-ce60c73e42e4"
  "resume_offset=13765705728"
];

This is an ext4 partition. Any ideas what I might be doing wrong?