Malformed binary after Attribute Certificate Table

[benthetechguy]

I'm trying to boot Ubuntu from iPXE using Ubuntu's signed shim. Here's what I'm trying to boot with:
kernel http://xxx.xxx.xxx.xxx/shimx64.signed.efi kernels/22.04.5/vmlinuz initrd=kernels/22.04.5/initrd insert cmdline here

Here's what I get when I run it:

http://xxx.xxx.xxx.xxx/shimx64.signed.efi... ok
Fetching Netboot Image http://xxx.xxx.xxx.xxx/kernels/22.04.5/vmlinuz
Malformed binary after Attribute Certificate Table
datasize: 16777216 SumOfBytesHashed: 11702784 SecDir->Size: 1928
hashsize: 5072504 SecDir->VirtualAddress: 0x00B29200
Failed to load image: Invalid Parameter
start_image() returned Invalid Parameter, falling back to default loader

Then it tries to load http://xxx.xxx.xxx.xxx/grubx64.efi which doesn't exist.

[benthetechguy]

This is also happening to me when I tell it to use actual GRUB from Ubuntu's /usr/lib/grub/x86_64-efi-signed/grubnetx64.efi.signed