So, this error means that you have a git repository inside your parent git repository, but there is no submodule information configured for it - i.e. you looked up a submodule (say "foo") and there is no configuration information for that submodule, but the submodule directory exists and contains a .git subdirectory (i.e. "parent/foo/.git" exists). It's a bit of an odd case but it would happen if you cloned into a parent and then tried to treat that clone as if it were a submodule without actually adding it.

It seems like this might be a misfeature and you could probably just treat this case the same as NOTFOUND (i.e. you tried to look up "xyz" as a submodule and it is not a submodule).