Async.runSync() returns error

When I use `Async.runSync(...)`, I get the following error on the client:

```
details: undefined
error: 500
errorType: "Meteor.Error"
message: "Internal server error [500]"
reason: "Internal server error"
```

When I switch back to `Meteor.sync(...)`, everything works perfectly again, even though `Meteor.sync()` is supposed to be deprecated. Can I continue to use `Meteor.sync()`?
