FreeBSD update cowardly refusing to proceed any further

I was recently upgrading one of my FreeBSD servers to 11-RELEASE and encountered an issue I haven’t run into in a while.

# freebsd-update upgrade -r 11.0-RELEASE
src component not installed, skipped
Looking up update.FreeBSD.org mirrors… 4 mirrors found.
Fetching metadata signature for 10.3-RELEASE from update6.freebsd.org… done.
Fetching metadata index… done.
Fetching 2 metadata patches.. done.
Applying metadata patches… done.
Fetching 1 metadata files… done.
Inspecting system… done.

The following components of FreeBSD seem to be installed:
kernel/generic world/base

The following components of FreeBSD do not seem to be installed:
world/doc world/games world/lib32

Does this look reasonable (y/n)? y

Fetching metadata signature for 11.0-RELEASE from update6.freebsd.org… done.
Fetching metadata index… done.
Fetching 1 metadata patches. done.
Applying metadata patches… done.
Fetching 1 metadata files… done.

The update metadata is correctly signed, but
failed an integrity check.
Cowardly refusing to proceed any further.

I have run into this in the past but I couldn’t remember what the solution was until I searched for it. Dan’s blog has the answer.