Getting 'badrequest' error deploying ARM

When I try to deploy an ARM template with an output:

```json
"outputs": {
"armkey": {
"type": "string",
// "value": "[listKeys(resourceId('Microsoft.Web/sites', variables('workerApp')), '2019-08-01')]"
// "value": "[listkeys(concat(resourceId('Microsoft.Web/sites', variables('workerApp')), '/host/default/'),'2016-08-01').functionKeys.default]"
"value": "[listkeys(concat(resourceId('Microsoft.Web/sites', variables('workerApp')), '/host/default/'),'2016-08-01').functionKeys.default]"
}
}
```

I'm receiving:

```json
{
"Code": "BadRequest",
"Message": "Encountered an error (ServiceUnavailable) from host runtime.",
"Target": null,
"Details": [
{
"Message": "Encountered an error (ServiceUnavailable) from host runtime."
},
{
"Code": "BadRequest"
},
{
"ErrorEntity": {
"Code": "BadRequest",
"Message": "Encountered an error (ServiceUnavailable) from host runtime."
}
}
],
"Innererror": null
}
```

No repository file, test, or entry point is named. Start by reproducing the ARM deployment with the provided output expressions and inspect the Azure Functions host-runtime response; done means the deployment succeeds and the output resolves without the BadRequest or ServiceUnavailable error.

Written by the indexing model from the issue text.