SparesMissing event on /dev/md4:nsa320 (fwd)

i upgraded an old nas device from debian 8 to 9 and started get'n this message
what's the dang deal :)
i don't have any spares


root@nsa320:~# mdadm -D /dev/md4
/dev/md4:
         Version : 0.90
   Creation Time : Tue Mar  5 13:16:13 2013
      Raid Level : raid1
      Array Size : 1932713920 (1843.18 GiB 1979.10 GB)
   Used Dev Size : 1932713920 (1843.18 GiB 1979.10 GB)
    Raid Devices : 2
   Total Devices : 2
Preferred Minor : 4
     Persistence : Superblock is persistent

     Update Time : Thu Jul  9 06:13:34 2020
           State : clean
  Active Devices : 2
Working Devices : 2
  Failed Devices : 0
   Spare Devices : 0

            UUID : dc6cc046:ef4012cc:ecea512d:0eb28517 (local to host nsa320)
          Events : 0.99097

     Number   Major   Minor   RaidDevice State
        0       8       20        0      active sync   /dev/sdb4
        1       8        4        1      active sync   /dev/sda4



---------- Forwarded message ----------
Date: Wed, 8 Jul 2020 18:20:14 -0500
From: mdadm monitoring <root-bi+AKbBUZKY6gyzm1THtWbp2dZbC/[email protected]>
To: [email protected]
Subject: SparesMissing event on /dev/md4:nsa320

This is an automatically generated mail message from mdadm
running on nsa320

A SparesMissing event had been detected on md device /dev/md4.

Faithfully yours, etc.

P.S. The /proc/mdstat file currently contains the following:

Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath]
md2 : active raid1 sda2[1] sdb2[0]
       19573696 blocks [2/2] [UU]

md4 : active raid1 sda4[1] sdb4[0]
       1932713920 blocks [2/2] [UU]

md3 : active raid1 sda3[1] sdb3[0]
       975808 blocks [2/2] [UU]

unused devices: <none>
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