Can't git stash after using git add -N

After using "git add -N", "git stash" can no longer stash:

/tmp/temp$ git init
Initialized empty Git repository in /tmp/temp/.git/
/tmp/temp$ echo a > a
/tmp/temp$ git add a
/tmp/temp$ git commit -m 'Initial commit of a'
[master (root-commit) d7242c4] Initial commit of a
 1 file changed, 1 insertion(+)
 create mode 100644 a
/tmp/temp$ echo b > b
/tmp/temp$ git add -N b
/tmp/temp$ git stash
error: Entry 'b' not uptodate. Cannot merge.
Cannot save the current worktree state

- Josh Triplett