AVAudioRecorder averagePowerForChannel always returns -120.0

I'm trying to use AVAudioRecorder's averagePowerForChannel method to monitor input levels on the microphone for an iPad/iPhone app. I have a callback which polls the average level in a loop — on the iPhone it works fine and returns sensible levels, but for some reason on the iPad it always returns -120.0.

Here's some of my setup code:

- (void) setupMic {
if (micInput) {
    [micInput release];
    micInput = nil;
}
NSURL *newURL = [[NSURL alloc] initFileURLWithPath:@"/dev/null"];

NSMutableDictionary *recordSettings = [[NSMutableDictionary alloc] init];

[recordSettings setObject:[NSNumber numberWithInt:kAudioFormatAppleLossless] forKey: AVFormatIDKey];
[recordSettings setObject:[NSNumber numberWithFloat:22050.0] forKey: AVSampleRateKey];
//  [recordSettings setObject:[NSNumber numberWithInt:2] forKey:AVNumberOfChannelsKey];
[recordSettings setObject:[NSNumber numberWithInt:12800] forKey:AVEncoderBitRateKey];
[recordSettings setObject:[NSNumber numberWithInt:16] forKey:AVLinearPCMBitDepthKey];
[recordSettings setObject:[NSNumber numberWithInt: AVAudioQualityLow] forKey: AVEncoderAudioQualityKey];

micInput = [[AVAudioRecorder alloc] initWithURL:newURL settings:recordSettings error:nil];
//  [micInput setMeteringEnabled:YES];

[newURL release];
[recordSettings removeAllObjects];
[recordSettings release];
}

As well as my start recording method:

- (void) startRecording {
NSLog(@"startRecording!");
[micInput pause];
[micInput prepareToRecord];
micInput.meteringEnabled = YES;
[micInput record];
[micInput updateMeters];
levelTimer = [[NSTimer alloc] initWithFireDate:[NSDate dateWithTimeIntervalSinceNow:0.0] interval:0.03 target:self selector:@selector(levelTimerCallback:) userInfo:nil repeats:YES];
[[NSRunLoop currentRunLoop] addTimer:levelTimer forMode:NSDefaultRunLoopMode];
}

and a bit of the levelTimer callback:

- (void)levelTimerCallback:(NSTimer *)timer {
[micInput updateM