"could not create work tree dir" with shell executor on Windows

The log output says it all.

gitlab-ci-multi-runner 0.3.2 (395f2e4)
Using Shell executor...

Running on <my-runner>...
Clonning repository...
fatal: could not create work tree dir 'tmp\builds\runner-be2b8409-project-1-concurrent-0\<gitlab-host>\<namespace>\<project>'.: No such file or directory
Das System kann den angegebenen Pfad nicht finden.
Checkouting c712aa29 as feature/stats-callbacks-events...
fatal: Not a git repository (or any of the parent directories): .git

Build failed with exit status 128

The one German line just means "The system cannot find the given path."

I'm on Windows 7, 32 bit, the runner is installed on C:\gitlab-ci-multi-runner.