Average bucket is not working

I use average bucket to calculate tps.

First, Working case
aggregation: average bucket
bucket: date histogram, field: @timestamp, minimum interval: second
metric: count

this case, is ok.
inspect:


The calculation process looks like below......

  1. count per bucket
  2. calculate average count
    3. average count / minimum interval: second(3h = 10800)

Second, Not Working case
aggregation: average bucket
bucket: date histogram, field: @timestamp, minimum interval: second
metric: unique count, field: primary key field

inspect:


The calculation process looks like below......
1.unique count per bucket
2. calculate average unique count

why don't calculate process 3???
(average unique count /minimum interval: second(3h = 10800)