Why can my custom-build iso not boot in EFI?

I am building a custom iso using xorriso in mkisofs mode. The build seems correct, inspecting the iso afterwards shows 2 boot images (1 BIOS, 2 UEFI), yet booting to the ISO from qemu using the OVMF firmware fails to find any bootable media. If I try the exact same boot in qemu but using a clean downloaded ubuntu 16.04 server iso, it boots without a problem. If I use BIOS (not UEFI), both boot just fine. xorriso command: xorriso -as mkisofs \ -l -J -R -V version \ -no-emul-boot -boot-load-size 4 -boot-info-table \ -b boot/isolinux/isolinux.bin -c boot/isolinux/boot.cat \ -isohybrid-mbr isohhdpfx.bin \ -eltorito-alt-boot -e EFI/BOOT/BOOTX64.EFI -no-emul-boot \ -isohybrid-gpt-basdat \ -o testos.iso ./cddir/ The details of examining the 2 iso (ubuntu and my custom built one) with xorriso -report_el_torito plain and cmd are in the following gist https://gist.github.com/deitch/e069268f92402d6a2b1c7e060ddba622