Cannot start Node.js function with debugger enabled

[jsumners-nr]

local.settings.json:

{
  "IsEncrypted": false,
  "Values": {
    "FUNCTIONS_WORKER_RUNTIME": "node",
    "AzureWebJobsStorage": "",
    "languageWorkers__node__arguments": "--inspect-brk"
  }
}

With that configuration set, func start will result in:

Starting inspector on 127.0.0.1:9229 failed: address already in use

If I set --inspect-brk=9330 then the same error occurs with a different port. How do I start this thing with the debugger available?