go: proxy.golang.org: '400 bad request' for pseudo-versions that refer to commits that also have canonical version tags

What version of Go are you using (go version)?

$ go version
go version go1.13beta1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

$ go env
GOHOSTARCH="amd64"
GOHOSTOS="linux"

Ubuntu Disco Dingo, go1.13beta1 downloaded from the Golang site.

What did you do?

go get -d github.com/future-architect/vuls
cd ~/go/src/github.com/future-architect/vuls
  # see that it has a `go.mod` already, so ..
go install

What did you expect to see?

Either a successful build or some kind of build failure

What did you see instead?

A bad request from proxy.golang.org:

go: github.com/tomoyamachi/reg@v0.16.2-0.20190418055600-c6010b917a55: reading https://proxy.golang.org/github.com/tomoyamachi/reg/@v/v0.16.2-0.20190418055600-c6010b917a55.mod: 400 Bad Request

I repeated the command, to rule out transient glitch. I then ran GOPROXY=direct go install which continued past that point to get to a different inscrutable error but one which is more likely tied to the project itself. Limiting this issue to the observed proxy error.