TypeError: unhashable type: 'GroundLit'

[tinicode]

I created my own test example to run inference with Gibbs Sampler. My python code is leaned on the given test file. But running the python script gives me the following error:

Traceback (most recent call last):
File "/Users/thaonguyen/Documents/Environments/pracmln_env/lib/python3.7/site-packages/pracmln/example/test_pet.py", line 38, in
main()
File "/Users/thaonguyen/Documents/Environments/pracmln_env/lib/python3.7/site-packages/pracmln/example/test_pet.py", line 33, in main
test_GibbsSampler()
File "/Users/thaonguyen/Documents/Environments/pracmln_env/lib/python3.7/site-packages/pracmln/example/test_pet.py", line 29, in test_GibbsSampler
multicore=True).run().write()
File "/Users/thaonguyen/Documents/Environments/pracmln_env/lib/python3.7/site-packages/pracmln/mlnquery.py", line 242, in run
inference = self.method(mrf, self.queries, **params)
File "/Users/thaonguyen/Documents/Environments/pracmln_env/lib/python3.7/site-packages/pracmln/mln/inference/gibbs.py", line 47, in init
for v in vars: self.var2gf[v].add(gf)
TypeError: unhashable type: 'GroundLit'_

If I use the smoking example instead of my own, the inference test works. Seems like something is wrong with my example. So I checked the syntax of the data files but couldn't find any mistakes...

I'd really appreciate your help. Many thanks in advance!

--> example attached
pracmln_pet_example.zip