return an empty array when using percentile

[ss666]

opentsdb version 2.4.1
HTTP requests were implemented in Python. Just wondering is there something wrong with data stored or query format, because query Q2 worked well, but query Q1 just returned an empty array []:

Q1. requests.get(OPENTSDB_QUERY_URL + 'start=' + str(start_time) + '&m=sum:percentiles[99.9,75.0] :cluster_allocatedVCores'+'&show_query=true', verify=False).json()

Q2. requests.get(OPENTSDB_QUERY_URL + 'start=' + str(start_time) + '&m=sum:cluster_allocatedVCores'+'&show_query=true', verify=False).json()