go mod tidy fails with latest version of a module

[Kaveh Shahbazian]

With this line in go.mod:

require github.com/labstack/echo v3.3.5

For go mod tidy it fails with:

go: finding github.com/labstack/echo v3.3.5
go: errors parsing go.mod:
/home/path-to/draftmod/go.mod:6: invalid module version "v3.3.5": unknown revision v3.3.5

But if the line get changed to:

require github.com/labstack/echo v3.0.0

Then go mod tidy works and the package dependency line, changes into:

github.com/labstack/echo v3.0.0+incompatible

Why?(Current module directory is outside `$GOPATH`)

[Kaveh Shahbazian]

echo has a version v3.1.0-rc.1 with a not standard format. That's why it works with v3.0.0+incompatible. Or at least that's my guess.

[Kaveh Shahbazian]

Got help on the matter.

Problem is with echo itself. After v3.2.2 they have dropped the letter v from tags.