Which version of the Serverless Framework are you using? Support for async functions when using invoke local is available since version 1.29.0 according to the changelog. Upgrading to a more recent version might help.
Another thing that comes to mind: according to the API Gateway documentation, the body property must be a JSON string when using the Lambda Proxy Integration (which is the case by default for the Serverless Framework). So maybe try this:
'body': JSON.stringify('Some Value')