How can I get AAC encoding with ExtAudioFile on iOS to work?

I need to convert a WAVE file into an AAC encoded M4A file on iOS. I'm aware that AAC encoding is not supported on older devices or in the simulator. I'm testing that before I run the code. But I still can't get it to work.

I looked into Apple's very own iPhoneExtAudioFileConvertTest example and I thought I followed it exactly, but still no luck!

Currently, I get a -50 (= error in user parameter list) while trying to set the client format on the destination file. On the source file, it works.

Below is my code. Any help is very much appreciated, thanks!

UInt32 size;

// Open a source audio file.
ExtAudioFileRef sourceAudioFile;
ExtAudioFileOpenURL( (CFURLRef)sourceURL, &sourceAudioFile );

// Get the source data format
AudioStreamBasicDescription sourceFormat;
size = sizeof( sourceFormat );
result = ExtAudioFileGetProperty( sourceAudioFile, kExtAudioFileProperty_FileDataFormat, &size, &sourceFormat );

// Define the output format (AAC).
AudioStreamBasicDescription outputFormat;
outputFormat.mFormatID = kAudioFormatMPEG4AAC;
outputFormat.mSampleRate = 44100;
outputFormat.mChannelsPerFrame = 2;

// Use AudioFormat API to fill out the rest of the description.
size = sizeof( outputFormat );
AudioFormatGetProperty( kAudioFormatProperty_FormatInfo, 0, NULL, &size, &outputFormat);

// Make a destination audio file with this output format.
ExtAudioFileRef destAudioFile;
ExtAudioFileCreateWithURL( (CFURLRef)destURL, kAudioFileM4AType, &outputFormat, NULL, kAudioFileFlags_EraseFile, &destAudioFile );

 // Create canonical PCM client format.
AudioStreamBasicDescription clientFormat;
clientFormat.mSampleRate = sourceFormat.mSampleRate;
clientFormat.mFormatID = kAudioFormatLinearPCM;
clientFormat.mFormatFlags = kAudioFormatFlagIsPacked | kAudioFormatFlagIsSignedInteger;
clientFormat.mChannelsPerFrame = 2;
clientFormat.mBitsPerChannel = 16;
clientFormat.mBytesPerFrame = 4;
clientFormat.mBytesPerPacket = 4;
clientFormat.mFramesPerPacket = 1;

// Set the client format in source and destination file.
size = sizeof( clientFormat );
ExtAudioFileSetProperty( sourceAudioFile, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat );
size = sizeof( clientFormat );
ExtAudioFileSetProperty( destAudioFile, kExtAudioFileProperty_ClientDataFormat, size, &clientFormat );

// Make a buffer
int bufferSizeInFrames = 8000;
int bufferSize = ( bufferSizeInFrames * sourceFormat.mBytesPerFrame );
UInt8 * buffer = (UInt8 *)malloc( bufferSize );
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0].mNumberChannels = clientFormat.mChannelsPerFrame;
bufferList.mBuffers[0].mData = buffer;
bufferList.mBuffers[0].mDataByteSize = ( bufferSize );

while( TRUE )
{
    // Try to fill the buffer to capacity.
    UInt32 framesRead = bufferSizeInFrames;
    ExtAudioFileRead( sourceAudioFile, &framesRead, &bufferList );

    // 0 frames read means EOF.
    if( framesRead == 0 )
        break;

    // Write.
    ExtAudioFileWrite( destAudioFile, framesRead, &bufferList );
}

free( buffer );

// Close the files.
ExtAudioFileDispose( sourceAudioFile );
ExtAudioFileDispose( destAudioFile );