Git ignores credential.helper setting

Question

For several months now, I've had to type in my credentials every time I want to push or pull from GitHub. I've tried several different credential helpers, but they all silently fail.

I believe I've found a clue as to why:

$ git config --add credential.helper fakecredentialhelper
$ git pull
Enter passphrase for key '/path/to/my/ssh_key': 
Already up to date.

According to the documentation, Git accepts any value for credential.helper and attempts to run that program. But fakecredentialhelper doesn't exist, meaning Git should get this result:

$ git credential-fakecredentialhelper --file=/mnt/shared/creds get
git: 'credential-fakecredentialhelper' is not a git command. See 'git --help'.

But calling git pull succeeds without complaint, meaning that Git didn't even attempt to run it. I'm entering my credentials; why is Git skipping credential.helper?

Or is Git swallowing the error?

^{Ubuntu 18.04.1 LTS
Git 2.17.1}