Git: “could not read from remote repository”, when remote is another local repo

I’m working in my local copy of a repository AAA, and I have added another local repository BBB as a remote bbb.
Specifically, I’ve tried adding it with each of

git remote add bbb ../../Path/to/BBB
git remote add bbb ../../Path/to/BBB/
git remote add bbb ../../Path/to/BBB/.git

However, with any of those, when I issue

git fetch bbb

it fails with error message

fatal: '../../Path/to/BBB' does not appear to be a git repository
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

The given path really is the root directory of a repository: if I issue

cd ../../Path/to/BBB
git status

git quite happily tells me about the status there. What could I be doing wrong?

(Background: I’m following the instructions in this answer for merging BBB into AAA.)

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