What is the problem?

Ray version and other system information (Python version, TensorFlow version, OS):
Ray: 0.8.2

Just recently a new check that hyperparam_mutations are either a list or a callable was introduced. (https://github.com/ray-project/ray/blob/master/python/ray/tune/schedulers/pbt.py#L182)

Unfortunately the configs I use with my Experiments are nested dicts and I can now no longer use PBT to tune parameters in those nested dicts.

Is there a reason that nested dicts are no longer allowed? :(