Setting rules-update-ssh-key results in "invalid format"

When configuring rules-update-ssh-key, I am seeing:

2023-05-08 18:20:22 WARNING unit.prometheus/0.config-changed logger.go:60 Cloning into '/var/snap/prometheus/current/custom.rules.d'...
2023-05-08 18:20:22 WARNING unit.prometheus/0.config-changed logger.go:60 Load key "/root/.ssh/prometheus_git_key": invalid format

This is preventing the ability to clone the repo. If I manually add the key to the machine and then do a diff between the two files I see:

root@prometheus-1:~/.ssh# diff prometheus_git_key prometheus_git_key_manual
38c38
< -----END OPENSSH PRIVATE KEY-----
\ No newline at end of file
---
> -----END OPENSSH PRIVATE KEY-----
root@prometheus-1:~/.ssh#

If I comment out the prometheus_ssh_keyfile portions of prometheus.py [1] and manually add the ssh key, adding the repo works fine.

[1] diff prometheus.py prometheus.py.new
157,159c157,159
< with open(PATHS["prometheus_ssh_keyfile"], "w") as f:
< f.write(config["rules-update-ssh-key"])
< os.chmod(PATHS["prometheus_ssh_keyfile"], 0o600)
---
> #with open(PATHS["prometheus_ssh_keyfile"], "w") as f:
> # f.write(config["rules-update-ssh-key"])
> #os.chmod(PATHS["prometheus_ssh_keyfile"], 0o600)
238,239c238,239
< if os.path.exists(PATHS["prometheus_ssh_keyfile"]):
< os.unlink(PATHS["prometheus_ssh_keyfile"])
---
> #if os.path.exists(PATHS["prometheus_ssh_keyfile"]):
> # os.unlink(PATHS["prometheus_ssh_keyfile"])