Empty local store + ssh extra store => "Protocol driver not attached"

[charles-dyfis-net]

I'm attempting to extract content with both a local store and a remote extra store. Thus, usage looks something like:

casync extract \
  --store=/local/path \
  --extra-store=server:/remote/path \
  test-blob.caibx \
  test-blob

If I leave out --store=/local/path and instead pass --store=server:/remote/path, then we do in fact connect to the server. With the remote repository only provided as an "extra" read-only store, however, the client never connects, and the following error is emitted:

Failed to run synchronizer: Protocol driver not attached