I've implemented a custom ContentProvider and am using MappedIdentity to map to external data, quite similar to this blog post by patkleef:
https://www.patrickvankleef.com/2016/12/11/ascend16-load-and-use-data-from-google-contacts-in-episerver/

All is well, I can map an external id to a ContentReference and the ContentProvider creates IContent properly.

However it seems to have broken the export/import data functionality in CMS/admin. Once I try to export a page with a ContentReference to my custom ContentProvider it breaks. 

I know there's a way to add DDS data to the export, as we're doing it for Geta Tags: https://github.com/Geta/Tags/blob/master/src/TagsTransferModule.cs#L21
However, I'm not storing data in DDS and the DynamicDataTransferHandler is the only TransferHandler that's configured (in Alloy).

Does anyone know how/if you can export MappedIdentities or if there's a different TransferHandler I can use?

The error I receive during export:
"The following errors have occurred:
[Exporting content 244] Exception: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index[]
[Exporting content 244] Can't export package because: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index"