weird error with serverless.yml variables

serverless.yml

service: testapi

vars:
   defaultRegion: us-east-1
   region: ${opt:region, self:vars.defaultRegion}
   stage: ${opt:stage, env:USER}

provider:
   name: aws
   runtime: nodejs4.3
   region: ${self:vars.region}
   stage: ${self:vars.stage}

functions:
   ping:
      handler: src/ping/Ping.handler
      memorySize: 128
      timeout: 2
      events:
         - http:
            method: GET
            path: ping