Models fail to generate output statistics

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Preparation

I installed hddm in a blank conda environment with Python 3.7 as recommended in the installation guide.
I followed the exact instructions specified in the repo's readme.

Code

The most minimal example where I managed to reproduce the error was training a simple HDDM model on my training data without any regression or depends_on arguments (But I have to mention that I encountered the same error trying to use HDDMRegressor and HDDMnnRegressor too).

import hddm
import pandas as pd

# This is a dataframe containing:
#   - subj_idx: str - subject ids
#   - rt: float - Reaction times in seconds
#   - response: float - 0.0 on incorrect responses, 1.0 on correct responses. 

dat = pd.read_csv("<filename>.csv")

model = hddm.HDDM(dat)
model.find_starting_values()
model.sample(1000, burn=1000)

Expected behaviour

The posterior is sampled without issue and output statistics are correctly calculated.

Actual behaviour

Sampling runs, but output statistics cannot be generated.
output:

/home/au689890/anaconda3/envs/hddm/lib/python3.7/site-packages/scipy/optimize/optimize.py:2215: RuntimeWarning: invalid value encountered in double_scalars
  tmp2 = (x - v) * (fx - fw)
 [-----------------100%-----------------] 1001 of 1000 complete in 80.5 sec
Could not generate output statistics for t
Could not generate output statistics for t_subj.<subject id>
...
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-4c77a0dec535> in <module>
      1 # %%
      2 ddm.find_starting_values()
----> 3 ddm.sample(1000, burn=1000)

~/anaconda3/envs/hddm/lib/python3.7/site-packages/kabuki/hierarchical.py in sample(self, *args, **kwargs)
    661         self.sampled = True
    662 
--> 663         self.gen_stats()
    664         return self.mc
    665 

~/anaconda3/envs/hddm/lib/python3.7/site-packages/kabuki/hierarchical.py in gen_stats(self, fname, print_hidden, **kwargs)
    739         """
    740 
--> 741         self.append_stats_to_nodes_db()
    742 
    743         sliced_db = self.nodes_db.copy()

~/anaconda3/envs/hddm/lib/python3.7/site-packages/kabuki/hierarchical.py in append_stats_to_nodes_db(self, *args, **kwargs)
    791             if self.nodes_db.loc[name, 'hidden']:
    792                 continue
--> 793             self.nodes_db.loc[name, 'mean']   = i_stats['mean']
    794             self.nodes_db.loc[name, 'std']    = i_stats['standard deviation']
    795             self.nodes_db.loc[name, '2.5q']   = i_stats['quantiles'][2.5]

TypeError: 'NoneType' object is not subscriptable