[BUG] Response Error Occurs from Python API SPARQL Query

[mengzhensun]

Describe the bug
The response error occurs from time to time when I queried with python api.

To Reproduce
Steps to reproduce the behavior:

1. Go to Colab Notebook
2. Run:

import datacommons as dc
root = 'https://api.datacommons.org'
query_str = '''
SELECT ?name ?pes_score
WHERE { 
    ?a typeOf HumanProteinOccurrence . 
    ?a proteinExpressionScore ?proteinExpressionScore .
    ?a name ?name .
    ?proteinExpressionScore name ?pes_score}LIMIT 2
'''
result = dc.query(query_str)

3. See response error in the screenshot sometimes

Expected behavior
It should return the query result as follows:
[{'?name': '4EBP3_HUMAN_Kidney_CellsInTubules',
'?pes_score': 'ProteinExpressionMedium'},
{'?name': '4ET_HUMAN_Breast_Adipocytes',
'?pes_score': 'ProteinExpressionMedium'}]

Screenshots
https://screenshot.googleplex.com/4d9f7313-5432-42d5-a07f-b478e8f134ab