I had added a new key to the secrets.yml file in our Rails application. The value of that key was derived from an ENV variable within an erb tag. In order to avoid a nil value when the ENV variable isn't present, which it may or may not be depending on the particular environment, I used the fetch method like so...

foo: <%= ENV.fetch('MY_VAR_NAME', '') %>

My intent was to have foo default to an empty string, but sadly it didn't. Once the erb was evaluated, it resulted in yaml that looked like this...

foo:

And of course, when that yaml is loaded, foo becomes nil. Not what I wanted. Instead, my yaml needed to look like this...

foo: ""

So I updated the secrets.yml file to look like this...

foo: <%= ENV.fetch('MY_VAR_NAME', '""') %>

And everything worked as expected.