db46 Checksum Error During MacPorts Upgrade

Problem

When upgrading my MacPorts installation, using sudo port upgrade outdated, I ran into the following error:

--->  Verifying checksum(s) for db46
Error: Checksum (md5) mismatch for patch.4.6.21.1
Error: Checksum (md5) mismatch for patch.4.6.21.2
Error: Checksum (md5) mismatch for patch.4.6.21.3
Error: Checksum (md5) mismatch for patch.4.6.21.4
***
The non-matching file appears to be HTML. See this page for possible reasons
for the checksum mismatch:

***
Error: Target org.macports.checksum returned: Unable to verify file checksums
Log for db46 is at: /opt/local/var/macports/logs/_opt_local_var_macports_sources_rsync.macports.org_release_ports_databases_db46/main.log
Error: Problem while installing db46

Solution

sudo port clean --all db46

Subsequently, running sudo port upgrade outdated worked as expected.