RAID10: wrong fs type, bad option, bad superblock

I'm trying to set up a RAID10 array by defining a RAID0 across one existing and one new RAID1 array.

The output from /proc/mdstat looks OK:

Personalities : [raid1] [raid0] [linear] [multipath] [raid6] [raid5] [raid4] [raid10]
md125 : active raid0 md126[1] md127[0]
      7813505024 blocks super 1.2 512k chunks

md126 : active (auto-read-only) raid1 sde1[0] sdd1[1]
      3906884608 blocks super 1.2 [2/2] [UU]
        resync=PENDING
      bitmap: 30/30 pages [120KB], 65536KB chunk

md127 : active (auto-read-only) raid1 sdb1[0] sdc1[1]
      3906884608 blocks super 1.2 [2/2] [UU]
        resync=PENDING
      bitmap: 29/30 pages [116KB], 65536KB chunk

unused devices: <none>

But when I try to mount /dev/md125, I get the dreaded error:

user@i7-harvester:~sudo mount /dev/md125 /mnt/plex
mount: /mnt/plex: wrong fs type, bad option, bad superblock on /dev/md125, missing codepage or helper program, or other error.

I've also added the output from fdisk -l, which does list a set of partitions on the /dev/md125 array, but I'm unsure how they got there or what that might signify:

user@i7-harvester:~$ sudo fdisk -l /dev/md*
fdisk: cannot open /dev/md: Is a directory


Disk /dev/md125: 7,28 TiB, 8001029144576 bytes, 15627010048 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 524288 bytes / 1048576 bytes


Disk /dev/md125p1: 1,87 TiB, 2028659590656 bytes, 3962225763 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 524288 bytes / 1048576 bytes
Alignment offset: 484352 bytes


Disk /dev/md125p2: 653,45 GiB, 701628183040 bytes, 1370367545 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 524288 bytes / 1048576 bytes
Alignment offset: 283136 bytes


Disk /dev/md125p4: 566,99 GiB, 608777137152 bytes, 1189017846 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 524288 bytes / 1048576 bytes
Alignment offset: 274944 bytes


Disk /dev/md126: 3,65 TiB, 4000649838592 bytes, 7813769216 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes


Disk /dev/md127: 3,65 TiB, 4000649838592 bytes, 7813769216 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

For posterity's sake. I didn't manage to grow the RAID10 config, so I completely formatted all four disks and went for a complex RAID10 setup.