JSON serialization error Object of type int32 is not JSON serializable

[zhubonan]

When I run the code below (code version 6cc8c38):

data= generate_slabs(
    structure='./POSCAR.mp-149_Si',
                    hkl=2,
                    parallelise=True,
                    #save_metadata=False,
                    thicknesses=[20,30], 
                    vacuums=[20,30], 
                    potcar_functional='ps', 
                    #save_slabs=False,
                    #save_metadata=False,
               make_input_files=False)

The following error is thrown:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-86e11d266020> in <module>
----> 1 data= generate_slabs(
      2     structure='./POSCAR.mp-149_Si',
      3                     hkl=2,
      4                     parallelise=True,
      5                     #save_metadata=False,

~/tmp/surfaxe/surfaxe/generation.py in generate_slabs(structure, hkl, thicknesses, vacuums, save_slabs, save_metadata, json_fname, make_fols, make_input_files, max_size, center_slab, ox_states, is_symmetric, layers_to_relax, fmt, name, config_dict, user_incar_settings, user_kpoints_settings, user_potcar_settings, parallelise, **kwargs)
    298             i['slab'] = i['slab'].as_dict()
    299         with open(json_fname, 'w') as f:
--> 300             json.dump(unique_list_of_dicts_copy, f)
    301 
    302     if save_slabs:

~/miniconda3/envs/py3w/lib/python3.8/json/__init__.py in dump(obj, fp, skipkeys, ensure_ascii, check_circular, allow_nan, cls, indent, separators, default, sort_keys, **kw)
    177     # could accelerate with writelines in some versions of Python, at
    178     # a debuggability cost
--> 179     for chunk in iterable:
    180         fp.write(chunk)
    181 

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode(o, _current_indent_level)
    427             yield _floatstr(o)
    428         elif isinstance(o, (list, tuple)):
--> 429             yield from _iterencode_list(o, _current_indent_level)
    430         elif isinstance(o, dict):
    431             yield from _iterencode_dict(o, _current_indent_level)

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_list(lst, _current_indent_level)
    323                 else:
    324                     chunks = _iterencode(value, _current_indent_level)
--> 325                 yield from chunks
    326         if newline_indent is not None:
    327             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    403                 else:
    404                     chunks = _iterencode(value, _current_indent_level)
--> 405                 yield from chunks
    406         if newline_indent is not None:
    407             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    403                 else:
    404                     chunks = _iterencode(value, _current_indent_level)
--> 405                 yield from chunks
    406         if newline_indent is not None:
    407             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_list(lst, _current_indent_level)
    323                 else:
    324                     chunks = _iterencode(value, _current_indent_level)
--> 325                 yield from chunks
    326         if newline_indent is not None:
    327             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    403                 else:
    404                     chunks = _iterencode(value, _current_indent_level)
--> 405                 yield from chunks
    406         if newline_indent is not None:
    407             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode_dict(dct, _current_indent_level)
    403                 else:
    404                     chunks = _iterencode(value, _current_indent_level)
--> 405                 yield from chunks
    406         if newline_indent is not None:
    407             _current_indent_level -= 1

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in _iterencode(o, _current_indent_level)
    436                     raise ValueError("Circular reference detected")
    437                 markers[markerid] = o
--> 438             o = _default(o)
    439             yield from _iterencode(o, _current_indent_level)
    440             if markers is not None:

~/miniconda3/envs/py3w/lib/python3.8/json/encoder.py in default(self, o)
    177 
    178         """
--> 179         raise TypeError(f'Object of type {o.__class__.__name__} '
    180                         f'is not JSON serializable')
    181 

TypeError: Object of type int32 is not JSON serializable

It is caused because the dictionary contains a int as keys. For python this is OK, but not the case for JSON where the keys must be strings. I am not able to pin point where the offending dictionary key is.