APT snapshot on proxy repo fails on empty metadata file

I defined an APT proxy repo, with the remote being the Postgres APT repo at http://apt.postgresql.org/pub/repos/apt/ and the distribution set to bionic-pgdg. I then attempted to snapshot the repo. This resulted in:

404 Content type could not be determined: dists/bionic-pgdg/13/binary-amd64/Packages

This is most likely because that file is currently empty (zero bytes).