Following the process explained in adb, I tried exporting a subfolder to the sdcard of my phone.

git annex initremote android type=adb androiddirectory=/sdcard/Music encryption=none exporttree=yes

The annex repository I am exporting from contains several subfolders with music albums. Using

git annex export master:album --to=android

results in the contents of the album folder going to the root Music folder on the remote, instead of in the Music/album folder.

How do I create a tree that contains the subfolder itself too?