AVAudioConverter Compressed to PCM conversion throwing OSStatus -50 (kAudio_ParamError)

I'm trying to use AVAudioConverter to convert compressed audio data (in this case mp3, 96kbps, CBR, stereo) into PCM data (the standard output format, obtained by calling audioEngine.outputNode.outputFormatForBus(0)). I use an AudioFileStream to get the packets. I have previously used an AudioQueue to play these packets, so they are valid.

This is where I call convertToBuffer:

let outputBuffer = AVAudioPCMBuffer(PCMFormat: outputFormat, frameCapacity: outputBufferFrameLength)
outputBuffer.frameLength = outputBufferFrameLength
var error:NSError?
audioConverter.convertToBuffer(outputBuffer, error: &error, withInputFromBlock: { (packetCount:AVAudioPacketCount, inputStatus:UnsafeMutablePointer<AVAudioConverterInputStatus>) -> AVAudioBuffer? in
    return self.getAudioConverterInput(packetCount, inputStatus: inputStatus)
})
if let error = error{
    print("conv error:", error)
}else{
    // TODO: Do stuff with buffer
}

And this is the function that handles the input.

func getAudioConverterInput(packetCount:AVAudioPacketCount, inputStatus:UnsafeMutablePointer<AVAudioConverterInputStatus>) -> AVAudioBuffer?{
        if let currentAudioFormat = currentAudioFormat, firstPackets = packets.first{
            inputStatus.memory = .HaveData
            let buffer = AVAudioCompressedBuffer(format: currentAudioFormat, packetCapacity: packetCount, maximumPacketSize: Int(currentMaximumPacketSize))
            var currentPackets:Packets = firstPackets
            var currentStartOffset:Int64 = 0
            for i in 0..<packetCount{
                let currentDescription = currentPackets.packetDescriptions[currentPacket]
                memcpy(buffer.data.advancedBy(Int(currentStartOffset)), currentPackets.data.advancedBy(Int(currentDescription.mStartOffset)), Int(currentDescription.mDataByteSize))
                buffer.packetDescriptions[Int(i)] = AudioStreamPacketDescription(mStartOffset: currentStartOffset, mVariableFramesInPacket: currentDescription.mVariableFramesInPacket, mDataByteSize: currentDescription.mDataByteSize)
            
                currentStartOffset += Int64(currentDescription.mDataByteSize)
                currentPacket+=1
            
                if (currentPackets.numberOfPackets == UInt32(currentPacket)){
                    currentPacket = 0
                    packets.removeFirst()
                    if let firstPackets = packets.first{
                        currentPackets = firstPackets
                    }else{
                        buffer.packetCount = i + 1
                        return buffer
                    }
                }
            }
            buffer.packetCount = packetCount
            return buffer
        }
        inputStatus.memory = .NoDataNow
        return nil
    }

This function is called once every time I call the convertToBuffer(...) function with the packetCount variable set to '1'. The error "conv error: Error Domain=NSOSStatusErrorDomain Code=-50 "(null)"" is thrown.

What am I doing wrong here?