Reproducible in a test repository with following steps:
$ touch untracked
$ git stash push -u -- untracked
Saved working directory and index state WIP on master: 0096475 init
fatal: pathspec 'untracked' did not match any files
error: unrecognized input
The file is stashed correctly, though.
Tested with Git 2.16.2 on Linux and Windows.
-Marc
