iOS microphone record driver caps

I'm trying to record from the microphone in iOS using the following procedure call:

 audio.clip = Microphone.Start("", false, 8, 44100);

The game starts up under the Unity editor without issue when I press the Play button, yet when I trigger the aforementioned code, I get the following error:

 Failed to get record driver caps. result=37 (An invalid paremeter was passed to this function. )
 UnityEngine.Microphone:Start(String, Boolean, Int32, Int32)

The microphone fails to record anything in this case.

I've checked the allowed sample rates, and both the minimum and maximum were 0, so I should be able to use any sample rate, according to the documentation I've read on the subject.

I ran this exact same game under Android in the Unity Editor (and as a built and exported app on actual Android devices), and there was no such issue. I have not tried building the iOS version.

How can I circumvent this error?