Can't find a package named like "$pkgname-client" in $depends

pmbootstrap seems to be unable to find a package if it's name includes variables like $pkgname. As an example: my rewrite of the upstream Snapcast APKBUILD contains 2 subpackages called $pkgname-client and $pkgname-server. While this seems perfectly valid, and Alpine Linux's Travis accepts it, pmbootstrap fails over it.

pmbootstrap log:

(007109) [22:29:59] Traceback (most recent call last):
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/__init__.py", line 61, in main
    getattr(frontend, args.action)(args)
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/helpers/frontend.py", line 104, in build
    args.strict, src=src):
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/build/_package.py", line 426, in package
    skip_init_buildenv, src):
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/build/_package.py", line 175, in init_buildenv
    depends, built = build_depends(args, apkbuild, arch, strict)
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/build/_package.py", line 130, in build_depends
    if package(args, depend, arch, strict=strict):
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/build/_package.py", line 417, in package
    apkbuild = get_apkbuild(args, pkgname, arch)
  File "/home/bart/Documents/Nextcloud/Projects/Personel/postmarketos-pmbootstrap/pmb/build/_package.py", line 67, in get_apkbuild
    raise RuntimeError("Package '" + pkgname + "': Could not find aport, and"
RuntimeError: Package '$pkgname-client': Could not find aport, and could not find this package in any APKINDEX!