reference a variable in serverless framework - serverless-framework

Question

I'm trying to define my serverless framework deployment bucket.
My serverless.yml looks like this:
provider:
name: aws
runtime: nodejs14.x
region: us-east-1
stage: dev
deploymentBucket:
name: ${self:environment.DEPLOYMENT_BUCKET}
environment:
${file(../evn.${opt:stage, 'dev'}.json)}
and the evn.dev.json file looks like this:
{
"DEPLOYMENT_BUCKET": "myBucketName"
}
(both of these files have non-relevant parts removed)
I'm getting a "cannot resolve variable at "provicer.deploymentBucket.name" error when trying to deploy.
How do I reference the DEPLOYMENT_BUCKET variable in the serverless.yml file?
EDIT: Other errors:
${environment}:DEPLOYMENT_BUCKET -> Could not locate deployment bucket. Error: The specified bucket is not valid
name: ${environment:DEPLOYMENT_BUCKET}1 -> Unrecognized configuration variable sources: "environment"
name: ${self:provider.environment:DEPLOYMENT_BUCKET}
and
name: ${self:environment:DEPLOYMENT_BUCKET}
-> Cannot resolve serverless.yml: Variables resolution errored with - Cannot resolve variable at "provider.deploymentBucket.name": Value not found at "self" source
I was able to solve the problem with this:
${file(../evn.${opt:stage, 'dev'}.json):DEPLOYMENT_BUCKET}
But 'reading' that file twice -- both here and in the 'environment' area seems to somewhat defeat the purpose of the environments area.