custom_registries ca_file docs misleading

The docs for adding a ca_file to custom_registries doesn't work when the file contains begin/end certificate lines

$ cat test1.pem
this is a test

$ juju config containerd custom_registries='[{"url": "https://registry.example.com", "ca_file": "'$(base64 < test1.pem)'"}]'

$ juju config containerd custom_registries
[{"url": "https://registry.example.com", "ca_file": "dGhpcyBpcyBhIHRlc3QK"}]

$ cat test2.pem
-----BEGIN CERTIFICATE-----
this is a test
this is also a test
another line
-----END CERTIFICATE-----

$ juju config containerd custom_registries='[{"url": "https://registry.example.com", "ca_file": "'$(base64 < test2.pem)'"}]'
ERROR cannot set and retrieve values simultaneously

Adding an extra set of quotes seems to properly escape the lines:
$ juju config containerd custom_registries='[{"url": "https://registry.example.com", "ca_file": "'"$(base64 < test2.pem)"'"}]'

$ juju config containerd custom_registries
[{"url": "https://registry.example.com", "ca_file": "LS0tLS1CRUdJTiBDRVJUSUZJQ0FURS0tLS0tCnRoaXMgaXMgYSB0ZXN0CnRoaXMgaXMgYWxzbyBh
IHRlc3QKYW5vdGhlciBsaW5lCi0tLS0tRU5EIENFUlRJRklDQVRFLS0tLS0K"}]