In hydra experiment when I want to sweep over two parameters, why does

hydra:
  mode: MULTIRUN
  sweeper:
      params:
        seed: 'range(1000,1005)'
        data.index: 'range(0,10)'

work, but

hydra:
  mode: MULTIRUN
  sweeper:
      params:
        seed: 'range(1000,1005)'
        data:
          index: 'range(0,10)'

doesn’t work? What is the underlying logic for dot index? Is this something specific to OmegaConf?