Clojurians Log v2

Not sure where to ask, maybe someone here can help. Very rarely, like once every few months, I see this exception:

heroku[web.1]: State changed from starting to up
app[web.1]: INFO  aleph.http.server [aleph-server-pool-4] - Setting up insecure HTTP/1 server pipeline.
app[web.1]: INFO  ring.logger [manifold-pool-2-1] - {:request-method :get, :uri "/ws/chet", :server-name "...", :ring.logger/type :starting}
app[web.1]: INFO  ring.logger [manifold-pool-2-1] - {:request-method :get, :uri "/ws/chet", :server-name "...", :ring.logger/type :params, :params {"client-id" "..."}}
app[web.1]: INFO  ring.logger [manifold-pool-2-1] - {:request-method :get, :uri "/ws/chet", :server-name "...", :ring.logger/type :finish, :status nil, :ring.logger/ms 61}
app[web.1]: INFO  aleph.http.server [aleph-server-pool-1] - Setting up insecure HTTP/1 server pipeline.
app[web.1]: INFO  ring.logger [manifold-pool-2-1] - {:request-method :get, :uri "/ws/chet", :server-name "...", :ring.logger/type :starting}
app[web.1]: INFO  ring.logger [manifold-pool-2-1] - {:request-method :get, :uri "/ws/chet", :server-name "...", :ring.logger/type :params, :params {"client-id" "..."}}
heroku[router]: at=info method=GET path="/ws/chet?client-id=..." host=... request_id=... fwd="..." dyno=web.1 connect=0ms service=15ms status=101 bytes=129 protocol=https
app[web.1]: WARN  io.netty.util.concurrent.DefaultPromise [aleph-server-pool-1] - An exception was thrown by io.netty.handler.codec.http.websocketx.WebSocketServerHandshaker$1.operationComplete()
java.util.NoSuchElementException: http-server
at io.netty.channel.DefaultChannelPipeline.getContextOrDie(DefaultChannelPipeline.java:1073)
at io.netty.channel.DefaultChannelPipeline.remove(DefaultChannelPipeline.java:423)
at io.netty.handler.codec.http.websocketx.WebSocketServerHandshaker$1.operationComplete(WebSocketServerHandshaker.java:235)
at io.netty.handler.codec.http.websocketx.WebSocketServerHandshaker$1.operationComplete(WebSocketServerHandshaker.java:230)
at io.netty.util.concurrent.DefaultPromise.notifyListener0(DefaultPromise.java:590)
at io.netty.util.concurrent.DefaultPromise.notifyListenersNow(DefaultPromise.java:557)
at io.netty.util.concurrent.DefaultPromise.access$200(DefaultPromise.java:35)
at io.netty.util.concurrent.DefaultPromise$1.run(DefaultPromise.java:503)
at io.netty.util.concurrent.AbstractEventExecutor.runTask(AbstractEventExecutor.java:173)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:166)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:470)
at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:413)
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
at manifold.executor$thread_factory$reify__1884$f__1885.invoke(executor.clj:71)
at clojure.lang.AFn.run(AFn.java:22)
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
at java.base/java.lang.Thread.run(Thread.java:1623)

And no complaints from any users, so no clue whether any requests are actually affected. Does anyone have any clue what's going on?