build command don't convert a image file to other style

[morimoto]

It cannot convert squashfs and writable image to another format.
When I do that, it shows message｀ERROR: Unsupported file type:｀.
However, only sandbox images could be converted to another format.

$ singularity shell test.simg
Singularity: Invoking an interactive shell within container...

Singularity test.simg:~/> exit
exit
$ sudo singularity build --writable test-w.simg test.simg
ERROR: Unsupported file type: test.simg
Cleaning up...

Build 2.4.2 from git repository source works as expected.

# lsb_release -rd
Description: Ubuntu Bionic Beaver (development branch)
Release: 18.04

# apt-cache policy singularity-container
singularity-container:
  Installed: 2.4.2-4
  Candidate: 2.4.2-4
  Version table:
 *** 2.4.2-4 500
        500 http://jp.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status