Post mileage - get error "category can't be blank"

In my post request I am defining category as either “Mileage” or “”, just as the documentation requires. I’m also posting all he other mandatory fields and some optional ones too.

Nevertheless I cannot shake this error. Example array being submitted:

[user] => 345673
[category] =>
[dated_on] => 2022-11-14
[description] => test trip
[project] => 3399083
[rebill_to_project] => 3399083
[mileage] => 30
[vehicle_type] => Car
[engine_type] => Electric
[engine_size] => All
[reclaim_mileage] => 1
[reclaim_mileage_rate] => 0.5
[rebill_mileage_rate] => 0.5

Is there something broken at FA or something missing in the documentation that’s causing this to fail? I’m out of ideas. Anyone seen this before?

I’ve solved it. When posting only one mileage claim the single expense must be nested inside the expenses endpoint.