Sync problem (Malformed YAML / undefined method 'to

I have an issue where I can deploy my site and it works perfectly in production, but on syncing the site locally I get the following error:

Malformed YAML in this file /.../Sites/Wagon/.../data/years.yml, error: undefined method 'to_sym' for 2014:Fixnum

The data/years.yml file looks like this:

- 2014:
    _slug: '2014'
    name: 2014
    projects:
    - residency-works

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