Error in 4-R2D2-Single

Hi,

I ran your code in 4-R2D2-Single, and got:
C:\POMDP\4-R2D2-Single\memory.py:88: RuntimeWarning: invalid value encountered in true_divide
prior_mean = abs_td_error_sum / lengths_burn
Traceback (most recent call last):
File "train.py", line 124, in
main()
File "train.py", line 100, in main
batch, indexes, lengths = memory.sample(batch_size)
File "C:\POMDP\4-R2D2-Single\memory.py", line 104, in sample
indexes = np.random.choice(range(len(self.memory_probability)), batch_size, p=probability)
File "mtrand.pyx", line 928, in numpy.random.mtrand.RandomState.choice
ValueError: probabilities contain NaN

Thanks!