genimage: no free space error for new partition

I'm trying to setup my buildroot image with genimage. This is the default configuration file:

image boot.vfat {
  vfat {
    files = {
      "bcm2710-rpi-3-b.dtb",
      "rpi-firmware/bootcode.bin",
      "rpi-firmware/cmdline.txt",
      "rpi-firmware/config.txt",
      "rpi-firmware/fixup.dat",
      "rpi-firmware/start.elf",
      "keel-marked/zImage"
    }
  }
  size = 32M
}

image sdcard.img {
  hdimage {
  }

  partition boot {
    partition-type = 0xC
    bootable = "true"
    image = "boot.vfat"
  }

  partition rootfs {
    partition-type = 0x83
    image = "rootfs.ext4"
  }
}

The nconfig > filesystem images fields are set to the default values. I don't need more freespace on rootfs.

Instead I want to create another ext4 partition, let's say of 100M. So this is the new genimage file I created:

image boot.vfat {
  vfat {
    files = {
      "bcm2710-rpi-3-b.dtb",
      "rpi-firmware/bootcode.bin",
      "rpi-firmware/cmdline.txt",
      "rpi-firmware/config.txt",
      "rpi-firmware/fixup.dat",
      "rpi-firmware/start.elf",
      "keel-marked/zImage"
    }
  }
  size = 32M
}

image opt.ext4 {
  ext4 { }
  size = 100M
}

image sdcard.img {
  hdimage {
  }

  partition boot {
    partition-type = 0xC
    bootable = "true"
    image = "boot.vfat"
  }

  partition rootfs {
    partition-type = 0x83
    image = "rootfs.ext4"
  }

  partition opt {
    partition-type = 0x83
    image = "opt.ext4"
  }
}

But it retus this error:

genext2fs: couldn't allocate a block (no free space)
ext4(opt.ext4): failed to generate opt.ext4

Where's my mistake? I think I don't need to reserve more space in the buildroot settings because I'm creating a new partition, I'm not resizing the rootfs one!