AVAssetExportSession fails if start time is not 0 - ios

Question

I am trying to export an audio file from the ipod library . If the start value is 0.0 then the export works just fine. If the start value is greater then 0.0 then the audio file is just empty. The status is completed but the file size is 0 kb . Can someone please give me a solution for this or is this another Apple bug ?
float vocalStartMarker = startValue;
float vocalEndMarker = endValue;
NSURL *audioFileInput = url;
NSString *voicePath = [path stringByAppendingPathComponent:#"audio"];
NSString *uniquePath = [mainViewController uniqueFilename:voicePath withExtension:#"caf"];
NSURL *audioFileOutput = [NSURL fileURLWithPath:uniquePath];
if (!audioFileInput || !audioFileOutput)
{
return NO;
}
[[NSFileManager defaultManager] removeItemAtURL:audioFileOutput error:NULL];
AVURLAsset *asset = [AVURLAsset URLAssetWithURL:audioFileInput options:nil];
AVAssetExportSession *exportSession = [AVAssetExportSession exportSessionWithAsset:asset
presetName:AVAssetExportPresetAppleM4A];
if (exportSession == nil)
{
return NO;
}
CMTime startTime = CMTimeMake((int)(floor(vocalStartMarker * 100)), 100);
CMTime stopTime = CMTimeMake((int)(ceil(vocalEndMarker * 100)), 100);
CMTimeRange exportTimeRange = CMTimeRangeFromTimeToTime(startTime, stopTime);
exportSession.outputURL = audioFileOutput;
exportSession.outputFileType = AVFileTypeAppleM4A;
exportSession.timeRange = exportTimeRange;
[exportSession exportAsynchronouslyWithCompletionHandler:^
{
if (AVAssetExportSessionStatusCompleted == exportSession.status)
{
NSLog(#"it worked:%#",uniquePath);
// It worked!
[music replaceObjectAtIndex:currentPage withObject:uniquePath];
}
else if (AVAssetExportSessionStatusFailed == exportSession.status)
{
NSLog(#"it failed");
// It failed...
}
}];
[self.popoverController dismissPopoverAnimated:YES];
self.popoverController=nil;
return YES;