[BUG] Query error: cell missing value {}

[mengzhensun]

Describe the bug
The python api throws the missing value error to my SPARQL query:

SELECT ?name ?proteinExpressionScore
WHERE {
  ?a typeOf HumanProteinOccurrence .
  ?a name ?name .
  ?a proteinExpressionScore ?proteinExpressionScore .
}
LIMIT 2

To Reproduce
Steps to reproduce the behavior:

1. Go to a Colab notebook
2. run

!pip install --upgrade --quiet datacommons
# Import Data Commons
import datacommons as dc
root = 'https://api.datacommons.org'

SELECT ?name ?proteinExpressionScore
WHERE {
  ?a typeOf HumanProteinOccurrence .
  ?a name ?name .
  ?a proteinExpressionScore ?proteinExpressionScore .
}
LIMIT 2

3. See error

Expected behavior

result = [{'?name': '2ABD_HUMAN_Pancreas_ExocrineGlandularCells', '?proteinExpressionScore': 'ProteinExpressionLow'},
 {'?name': '2A5G_HUMAN_Bronchus_RespiratoryEpithelialCells', '?proteinExpressionScore': 'ProteinExpressionMedium'}]

Screenshots
Missing value error: https://screenshot.googleplex.com/fa799d8a-84fb-4972-a73d-c680861d19af
Response error: https://screenshot.googleplex.com/54176345-fe07-4c5d-91aa-01536ab01141
Additional context
Response error occurs sometimes for the same query as well, as shown in the second screenshoot.