Hydra: How to express None in config files?

I have a very simple Python script:

import hydra
from omegaconf import DictConfig, OmegaConf


@hydra.main(version_base="1.3", config_path=".", config_name="config")
def main(cfg: DictConfig) -> None:
    if cfg.benchmarking.seed_number is None: 
        raise ValueError()

if __name__ == "__main__": 
    main()

And here the config file:

benchmarking: 
  seed_number: None 

Unfortunately, the Python script does not raise an error. Instead, when I print the type of cfg.benchmarking.seed_number, it is str. How can I pass None instead?

Try null:

benchmarking: 
  seed_number: null