Bug Report: Custom ticket fields are not being exported in the JSON export

We are trying to perform a full JSON export of our ticket data for our internal data warehouse. We've noticed that the export does not include the values for any of our custom ticket fields. The standard fields are all there, but the custom fields are completely missing from the JSON objects. We have confirmed that the custom fields have data in them by looking at the tickets in the agent interface. This is a major issue for us, as our reporting heavily relies on the data in these custom fields. Is this a known bug with the JSON export, or is there an additional parameter we need to include to get the custom field data?