mkisofs error - boot image...efibot.img not an allowable size

I am trying to create a custom ISO from a rhel-8 installation disk that can boot on either a BIOS or an EFI server. Everything goes fine until I try to create the iso. If I run the following command:

mkisofs -J -R -T -V "NGS-8.4-0 Server" \
    -o ngs-8.4-0.iso \
    -b isolinux/isolinux.bin \
    -c isolinux/boot.cat \
    --no-emul-boot \
    --boot-load-size 4 \
    --boot-info-table \
    --eltorito-alt-boot \
    -e images/efiboot.img \
    -m TRANS.TBL \
    ngs-dvd

I get the following output:

Creating NGS iso...I: -input-charset not specified, using iso-8859-1 (detected in locale settings)

(bunch of TRANS.TBL output deleted)

Size of boot image is 4 sectors -> No emulation
Size of boot image is 19612 sectors -> genisoimage: Error - boot image '/NGS/ngs-dvd/images/efiboot.img' has not an allowable size.

However, if I delete two options (--eltorito-alt-boot & -e images/efiboot.img), it creates a bootable iso. What am I doing wrong?