Websocket invalid frame header

I am sending data from a java server to a javascript client via a websocket in the following manner:

private byte[] makeFrame(String message) throws IOException {    byte[] bytes = message.getBytes(Charset.forName("UTF-8"));    ByteArrayOutputStream byteStream = new ByteArrayOutputStream();    byteStream.write(0x81);    byteStream.write(bytes.length);    byteStream.write(bytes);    byteStream.flush();    byteStream.close();    byte[] data = byteStream.toByteArray();}

But i am getting the error

Websocket connection to 'ws://localhost:8080/' failed: Invalid frame header

when the size is large (i believe above 128 bytes). I am unsure whether this is an issue with the op-code or something else.

Many thanks, Ben