请问 docker save 为何出现 could not verify layer data 的错误提示，从而无法备份？

我 pull 了这个 quay.io/travisci/travis-ruby， 装了点东西然后 commit 了一下， 为何 save 的时候提示

$ sudo docker save -o tt.tar.gz tt:latest
Error response from daemon: could not verify layer data for: sha256:xxxxx. This may be because internal files in the layer store were modified. Re-pulling or rebuilding this image may resolve the issue

问了下 stackoverflow 还没答案 https://stackoverflow.com/questions/45535790/how-do-i-use-docker-save-to-backup-a-local-image-to-file/