git sub-module references unknown commit

[nivekastoreth]

WSL Ubuntu:

jmaki@jmaki-pc /build/public/DiscordBot/src
$ git --version
git version 2.7.4

jmaki@jmaki-pc /build/public/DiscordBot
$ git submodule update --init --recursive
Submodule 'Discord.Net' (https://github.com/devinvisible/Discord.Net.git) registered for path 'Discord.Net'
Cloning into 'Discord.Net'...
remote: Counting objects: 27339, done.
remote: Compressing objects: 100% (2/2), done.
remote: Total 27339 (delta 0), reused 0 (delta 0), pack-reused 27337
Receiving objects: 100% (27339/27339), 6.02 MiB | 1.76 MiB/s, done.
Resolving deltas: 100% (20365/20365), done.
Checking connectivity... done.
fatal: reference is not a tree: 624b2d553b42acda7c4fe884d8d276498b9cf6f5
Unable to checkout '624b2d553b42acda7c4fe884d8d276498b9cf6f5' in submodule path 'Discord.Net'

Cygwin:

jmaki@jmaki-pc /build/public/DiscordBot
$ git --version
git version 2.16.2

jmaki@jmaki-pc /build/public/DiscordBot
$ git submodule update --init --recursive
error: Server does not allow request for unadvertised object 624b2d553b42acda7c4fe884d8d276498b9cf6f5
Fetched in submodule path 'Discord.Net', but it did not contain 624b2d553b42acda7c4fe884d8d276498b9cf6f5. Direct fetching of that commit failed.