Using env var on serverless.test.yaml

Serverless Framework
1

Hi
Im successfully using environment variables on my serverless.yaml file with:
${env:VAR_NAME}

But,
This solution doesnt work on serverless.test.yaml. I am testing there apis which are protected with api key.
Attempt to define that secret x-api-key doesnt work for me (getting FORBIDDEN 403):
request:
headers:
x-api-key: ${env:SLS_DEV_API_KEY}

running from the CLI:
SLS_DEV_API_KEY=SECRET sls test

If I use the actual key string it works, so I assume I am not reading the environment var correctly.
Would appreciate your help