Submodule strategy fails

[lightheaded]

I have a submodule build initialized at the root of my dir and I'm trying to deploy with this conf

activate :deploy do |deploy|
  deploy.method   = :git
  deploy.strategy = :submodule
  deploy.remote   = 'origin'
  deploy.branch   = 'master'
end

And the output is

## Deploying via git to remote="origin" and branch="master"
Already on 'master'
No stash found.
Can't deploy! Please resolve conflicts. Then process to manual commit and push on master branch.

If I look at the source, I see that stashes are involved. What exactly is stash used for? If I have previously updated everything and root and submodule repos are clean then there is nothing to stash and pop. Am I right or I'm getting something fundamentally wrong?