genisoimage fails missing pathspec

Issue

• When trying to create an ISO image, I get a "missing pathspec" error

  # genisoimage -U -r -v -T -J -joliet-long -V "RHEL-7.4 Server.x86_64" -volset "RHEL-7.4 Server.x86_64" -A "RHEL-7.4 Server.x86_64" -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 -no-emul-boot -o ../RHEL7.iso
   Warning: creating filesystem that does not conform to ISO-9660. I: -input-charset not specified, using utf-8 (detected in locale settings) genisoimage 1.1.11 (Linux) genisoimage: Missing pathspec. Usage: genisoimage [options] -o file directory ...