Help needed: How to divide two value stream values

Hello,

how can I divide two measurements and display the ratio between them? I tried the following but it does not work:

inbytes  =from(bucket: "kafka")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "kafka_server_name_BytesInPerSec_type_BrokerTopicMetrics" )
  |> filter(fn: (r) => r["_field"] == "value")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")

outbytes  =from(bucket: "kafka")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "kafka_server_name_BytesOutPerSec_type_BrokerTopicMetrics")
  |> filter(fn: (r) => r["_field"] == "value")
  |> aggregateWindow(every: v.windowPeriod, fn: mean, createEmpty: false)
  |> yield(name: "mean")

inbytes / outbytes

Can you expand on what tje above means? Does not work

Also do each queries produce any value? Could one or the other be null or 0?