Allow for failed checkout in Azure Devops YAML

I want to conditionally checkout branch B if unable to checkout branch A (e.g. it doesn't exist).

      - checkout: self
      - checkout: git://stackoverflow/_git/MuhRepo@A
        continueOnError: true
      - checkout: git://stackoverflow/_git/MuhRepo@B
        condition: failed()

Unfortunately, rather than running through the pipeline is blocked with:

This pipeline is not valid. Could not get the latest source version for repository MuhRepo hosted on Azure Repos using ref refs/head/A

I would rather avoid calling a bash script that does the git stuff for me as it would seem against the spirit of devops.

Any advice appreciated!