Generating init with 6.18 kernel tree

When generating init from the linux-next branch - current 6.18rc4, something is causing mkinitcpio to throw the error

/usr/bin/mkinitcpio: line 780: _addedmodules[${modname//-/_}]: bad array subscript

The complete output

[root@archlinux mkinitcpio.d]# mkinitcpio -p linux-next-git
==> Building image from preset: /etc/mkinitcpio.d/linux-next-git.preset: 'default'
==> Using default configuration file: '/etc/mkinitcpio.conf'
 -> -k /boot/vmlinuz-linux-next-git -U /efi/EFI/Linux/arch-linux-next-git.efi --splash /usr/share/systemd/bootctl/splash-arch.bmp
==> Starting build: '6.18.0-rc4-next-20251105-1-next-git-06507-g84d39fb9d529'
/usr/bin/mkinitcpio: line 780: _addedmodules[${modname//-/_}]: bad array subscript
 -> Running build hook: [systemd]
 -> Running build hook: [plymouth]
 -> Running build hook: [kms]
==> WARNING: Possibly missing firmware for module: 'ast'
==> WARNING: Possibly missing firmware for module: 'i915'
==> WARNING: Possibly missing firmware for module: 'nouveau'
==> WARNING: Possibly missing firmware for module: 'radeon'
==> WARNING: Possibly missing firmware for module: 'xe'
 -> Running build hook: [modconf]
 -> Running build hook: [keyboard]
==> WARNING: Possibly missing firmware for module: 'xhci_pci_renesas'
 -> Running build hook: [block]
==> WARNING: Possibly missing firmware for module: 'aic94xx'
==> WARNING: Possibly missing firmware for module: 'bfa'
==> WARNING: Possibly missing firmware for module: 'isci'
==> WARNING: Possibly missing firmware for module: 'qed'
==> WARNING: Possibly missing firmware for module: 'qla1280'
==> WARNING: Possibly missing firmware for module: 'qla2xxx'
==> WARNING: Possibly missing firmware for module: 'wd719x'
==> WARNING: Possibly missing firmware for module: 'cs42l43'
 -> Running build hook: [filesystems]
 -> Running build hook: [btrfs]
==> WARNING: Possibly missing '/usr/bin/ash' for script: /usr/lib/initcpio/hooks/btrfs
 -> Running build hook: [sd-vconsole]
 -> Running build hook: [sd-encrypt]
==> WARNING: Possibly missing firmware for module: 'qat_420xx'
==> WARNING: Possibly missing firmware for module: 'qat_4xxx'
==> WARNING: Possibly missing firmware for module: 'qat_c3xxx'
==> WARNING: Possibly missing firmware for module: 'qat_c62x'
==> WARNING: Possibly missing firmware for module: 'qat_dh895xcc'
 -> Running build hook: [fsck]
==> WARNING: Possibly missing '/bin/sh' for script: /usr/bin/fsck.btrfs
==> WARNING: Possibly missing '/bin/sh' for script: /usr/bin/fsck.btrfs
==> Generating module dependencies
==> Creating zstd-compressed initcpio image
 -> Early uncompressed CPIO image generation successful
==> Initcpio image generation successful
==> Creating unified kernel image: '/efi/EFI/Linux/arch-linux-next-git.efi'
 -> Using ukify to build UKI
Using config file: /usr/lib/kernel/uki.conf
Wrote unsigned /efi/EFI/Linux/arch-linux-next-git.efi
==> Unified kernel image generation successful
==> Running post hooks
 -> Running post hook: [sbctl]
Signing /efi/EFI/Linux/arch-linux-next-git.efi
✓ Signed /efi/EFI/Linux/arch-linux-next-git.efi
==> Post processing done

• mkinitcpio 39
• mkinitcpio 40 from core-testing
• systemd 258.1