createMeta not returning create fields for an issue type

Hi, Is there any reason that createMeta would not return any create issue fields for a particular issue type? For one of our customers, their sub-task issue type returns an empty array as the list of fields. This uses the createMeta REST API and only occurs for that issue type, not the others

Any ideas?

EDIT: I using expand=projects.issuetypes.fields in the URL