Using the following contents saved in test.yml:
If you paste the same contents into http://www.yamllint.com/, it produces the correct error message:
Evergreen's validator references the wrong line number. This is a more significant issue on actual, non-trivial project files because when neither of the two lines is simple one can spend a bit of time trying to find an error when none exists.
(I upgraded the evergreen binary prior to reporting this.)