EOFException: eof unexpected occurred while writing to channel null

[pimonnat]

I test the api and got the error 

{
     "fault": {
          "faultstring": "Unexpected EOF at target",
           "detail": {
                  "errorcode": "messaging.adaptors.http.flow.UnexpectedEOFAtTarget"
            }
     }

}

And in trace session I see this information

Tree: TARGET_SELECTOR
error: Unexpected EOF at target
error.cause: eof unexpected
error.class: com.apigee.errors.http.server.BadGateway
Identifier: fault
state: RESP_START
type: ErrorPoint

So I try to find the cause of this error in system.log and then see this message

NIOThread@2 INFO HTTP.SERVICE - ExceptionHandler.handleException() : Exception java.io.EOFException: eof unexpected occurred while writing to channel null
java.io.EOFException: eof unexpected
java.io.EOFException: eof unexpected

But I'm not sure what I should do next. 

What is possible cause of "eof unexpected occurred while writing to channel null" ??