They mean trouble anyway, but here’s a new trick that Git has up it’s sleeve. The problem appears when a submodule does not have a master branch, because you named (or someone else who created the submodule) the branch main, as everyone says you should do these days. The error you get contains the sarcastic sentence:

Thanks for nothing. The fix is to remove all traces of the submodule (if you don’t, re-cloning will fail) and re-clone, naming a branch with the -b option. Here we go (assuming we are on the top level of the master^[1] repo):

Solution found here: https://stackoverflow.com/questions/11887203/you-are-on-a-branch-yet-to-be-born-when-adding-git-submodule.