Derivative platform - cannot set unit_of_measurement

Configuration
1

Hi,
I have this in my config:

sensor:
  - platform: derivative
    name: Hive Heating Temp derivative
    source: sensor.hive_heating_reported_current_temp
    unit: °C/Hr
    time_window: 00:05:00

However, the resulting sensor has a unit_of_measurement of °C/min and should be °C/Hr

I have also tried:

sensor:
  - platform: derivative
    name: Hive Heating Temp derivative
    source: sensor.hive_heating_reported_current_temp
    unit_time: h
    time_window: 00:05:00

and

sensor:
  - platform: derivative
    name: Hive Heating Temp derivative
    source: sensor.hive_heating_reported_current_temp
    time_window: 00:05:00

but no joy.

sensor.hive_heating_reported_current_temp has a unit_of_measurement of °C.

What am I missing?