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Fix Netlify CMS YAML error 'Implicit map keys need to be on a single line'

When trying to setup Netlify CMS on a project, I kept getting this error on the /admin page:

Error loading the CMS configuration
Config Errors:

YAMLSemanticError: Implicit map keys need to be on a single line at line 1, column 1:

<!DOCTYPE html>

Check your config.yml file.

I checked and doubled checked the YAML in my config.yml file, and everything was valid. I went through the Netlify Identity steps to verify that they were followed correctly, but that didn’t help either.

Googling the error message brought up nothing useful.

Then it occurred to me: “What if Netlify CMS isn’t getting the config file at all?”

Sure enough, my static-site generator was skipping the unrecognized .yml file when building the site. I added the configuration to copy that file to the build folder, and voilà: everything works as expected.

So if you are getting the same error, check to make sure that the config.yml file is making it to your built admin/ directory.

Happy debugging!