What is the error type of this error so I can ignore it?

Here is an example of an error type within this library.

closeErr := new(websocket.CloseError)
if errors.As(err, closeErr) {
...
}

websocket.CloseError is an error type within this library.

So, what error type is returned when you receive the error failed to close WebSocket: received header with unexpected rsv bits set: true:true:true?