GeoServer - Ingest not supported for mosaics

GeoServer 2.18.0, Java 1.8 on Windows.

I have succeeded in creating an ImageMosaic store using the REST API.

This store is located as a single directory in the file system. The data is GeoTIFF.

I would like to add new tiles to this store. To do this I add a targetStore JSON object to the REST API import object. GeoServer returns an INIT_ERROR with a message that "ingest not supported for mosaics".

This seems to indicate that it is not possible to extend the mosaic once created. Is this the case, or maybe I am calling the API incorrectly?