Prioritized Replay Buffer Demo Code Not Working

from cpprb import PrioritizedReplayBuffer

buffer_size = 256

prb = PrioritizedReplayBuffer(buffer_size,
                              {"obs": {"shape": (4,4)},
                               "act": {"shape": 3},
                               "rew": {},
                               "next_obs": {"shape": (4,4)},
                               "done": {}},
                              alpha=0.5)

for i in range(1000):
    prb.add(obs=np.zeros((4,4)),
            act=np.ones(3),
            rew=0.5,
            next_obs=np.zeros(4,4),
            done=0)

batch_size = 32
s = prb.sample(batch_size,beta=0.5)

indexes = s["indexes"]
weights = s["weights"]

#  train
#  ...


per.update_priorities(indexes,new_priorities)

gets the output of

Traceback (most recent call last):

  File "<ipython-input-13-0af7c3627486>", line 17, in <module>
    next_obs=np.zeros(4,4),

TypeError: data type not understood