AVAudioRecorder - Why applying 32000 AVEncoderBitRateKey for a 8000 AVSampleRateKey will cause AudioCodecInitialize failed?

Question

Currently, I want to setup a reasonable quality sound recording, which consumes minimal disk space.

I try to use the following setup

1. AVSampleRateKey: 8000 - "Telephone and encrypted walkie-talkie, wireless intercom and wireless microphone transmission; adequate for human speech but without sibilance (ess sounds like eff (/s/, /f/))." From https://en.wikipedia.org/wiki/Sampling_(signal_processing)
2. AVEncoderBitRateKey: 32000 - "32 kbit/s – generally acceptable only for speech". From https://en.wikipedia.org/wiki/Bit_rate

If I use the following settings for AVAudioRecorder

let settings = [
    AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
    AVNumberOfChannelsKey: 1,
    AVSampleRateKey: 8000,
    AVEncoderBitRateKey: 32000,
    AVEncoderAudioQualityKey: AVAudioQuality.high.rawValue
]

It will cause AudioCodecInitialize failed error during avAudioRecorder.record()

My current workaround is to eliminate AVEncoderBitRateKey, and apply the following settings.

let settings = [
    AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
    AVNumberOfChannelsKey: 1,
    AVSampleRateKey: 8000,
    AVEncoderAudioQualityKey: AVAudioQuality.high.rawValue
]

However, I still would like to know, why 32000 AVEncoderBitRateKey will fail, and is there any way I can further optimize, to achieve reasonable quality and minimal disk space, for sound recording?