Substract host name from FQDN in flux filter stop influxdb

Fluxlang
,
1

Hi, i’m try to filter query data by part of tag value. Extract host part of FQDN and compare with string/variable.

import "strings"

from(bucket: "vCenter")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "vsphere_vm_cpu")
  |> filter(fn: (r) => r["_field"] == "usagemhz_average")
  |> filter(fn: (r) => strings.substring(end: strings.index(substr:"." , v:r.guesthostname ), start:0 , v:r.guesthostname ) == "ada")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")

after this query stop with “panic: runtime error: slice bounds out of range [:-1]”

I use influxdb 2.1.1

What am I doing wrong ?