writeDataUnsupported in ChannelInboundHandler (Swift-NIO)

I am trying to make a simple echo UDP server that sends back all incoming datagrams prefixed with a UTF8 string.

In my attempts to reach this goal, I succeeded in sending back the incoming data, but when I try to prefix this data with the string: "You sent: ", I get an error writeDataUnsupported

This is my code:

I made a ChannelInboundHandler called Echo all it does is: For each incoming datagram, it sends the string "You sent: " and then the data of the incoming datagram.

final class Echo: ChannelInboundHandler {
    typealias   InboundIn = ByteBuffer
    typealias OutboundOut = ByteBuffer

    var wroteResponse = false
    static let response = "You sent: ".data(using: .utf8)!

    func channelRead(ctx: ChannelHandlerContext, data: NIOAny) {
        if !wroteResponse {
            var buffer = ctx.channel.allocator.buffer(capacity: Echo.response.count)
            buffer.write(bytes: Echo.response)
            ctx.write(self.wrapOutboundOut(buffer), promise: nil)
            wroteResponse = true
        ctx.write(data, promise: nil)

    func channelReadComplete(ctx: ChannelHandlerContext) {
        wroteResponse = false

Then I made a single threaded event loop group and assigned a datagram bootsrap to it. Then I bound the bootstrap to port 4065.

let 🔂 = MultiThreadedEventLoopGroup(numThreads: 1)
let bootstrap = DatagramBootstrap(group: 🔂)
    .channelOption(ChannelOptions.socket(SocketOptionLevel(SOL_SOCKET), SO_REUSEADDR), value: 1)
    .channelInitializer { $0.pipeline.add(handler: Echo()) }
defer {
    try! 🔂.syncShutdownGracefully()

try bootstrap
    .bind(host: "", port: 4065)

Why do I always get this writeDataUnsupported while trying to send the string: "You sent: "?