Travis_setup_go exits with 86

I have a pretty simple .travis.yml file for building a Go project on Linux, macOS and Windows:

language: go
go:
    - 1.x
    - master
env:
    - GO111MODULE=on
os:
    - linux
    - osx
    - windows
matrix:
    allow_failures:
        - os: windows
before_script: 
    - if [ "$TRAVIS_OS_NAME" = "windows" ]; then choco install make ; fi
script:
    - make test

On macOS and Linux the builds succeed but on Windows the build only succeeds for Go 1.x, not for master. Here’s the error message:

$ travis_setup_go
The command "travis_setup_go" failed and exited with 86 during .

Your build has been stopped.
I don't have any idea what to do with 'master'.
(using download type 'auto')
Failed to run gimme

So what is going wrong there?