pmseries rate() doesn't work

[andreasgerstmayr]

without rate():

$ curl -s "http://localhost:44322/series/values?start=$(( $(date +%s)-60 ))&finish=$(date +%s)&interval=15&series=$(pmseries "kernel.all.sysfork")" | jq
[
  {
    "series": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730856274.284,
    "value": "1750473"
  },
  {
    "series": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730866301.775,
    "value": "1750499"
  },
  {
    "series": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730876305.622,
    "value": "1750516"
  },
  {
    "series": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730896279.187,
    "value": "1750870"
  },
  {
    "series": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730906300.683,
    "value": "1750953"
  }
]

with rate():

$ curl -s "http://localhost:44322/series/values?start=$(( $(date +%s)-60 ))&finish=$(date +%s)&interval=15&series=$(pmseries "rate(kernel.all.sysfork)")" | jq
[
  {
    "series": "96f0063468e22d03e846241ad3cb44dda68a5a61",
    "instance": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730846304.80,
    "value": "1750463"
  },
  {
    "series": "96f0063468e22d03e846241ad3cb44dda68a5a61",
    "instance": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730866301.775,
    "value": "1750499"
  },
  {
    "series": "96f0063468e22d03e846241ad3cb44dda68a5a61",
    "instance": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730876305.622,
    "value": "1750516"
  },
  {
    "series": "96f0063468e22d03e846241ad3cb44dda68a5a61",
    "instance": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730896279.187,
    "value": "1750870"
  },
  {
    "series": "96f0063468e22d03e846241ad3cb44dda68a5a61",
    "instance": "0da9ac73a95ada435faa8e1d1afdd73e0370d2d9",
    "timestamp": 1636730906300.683,
    "value": "1750953"
  }
]

There's no rate conversation performed.