panic: persistConn was already in LRU

goroutine 609111 [running]:
panic(0xa50dc0, 0x125a45d8)
        /opt/dev/gopkg/go/src/runtime/panic.go:500 +0x331
net/http.(*connLRU).add(0x125de5b4, 0x1267c510)
        /opt/dev/gopkg/go/src/net/http/transport.go:2147 +0x177
net/http.(*Transport).tryPutIdleConn(0x125de5a0, 0x1267c510, 0x0, 0x0)
        /opt/dev/gopkg/go/src/net/http/transport.go:678 +0x5d8
net/http.(*persistConn).readLoop.func2(0x0, 0x125f2b01)
        /opt/dev/gopkg/go/src/net/http/transport.go:1391 +0x32
net/http.(*persistConn).readLoop(0x1267c510)
        /opt/dev/gopkg/go/src/net/http/transport.go:1548 +0xce1
created by net/http.(*Transport).dialConn
        /opt/dev/gopkg/go/src/net/http/transport.go:1062 +0xda5

func DoRequest(/* som arguments */) {
        // Very high frequency call code
	var res *http.Response
	err = nil
	if r.Timeout > 0 {
		err = errors.New("timeout")
		c := make(chan *httpPeek)
		go func() {
			newRes, newErr := httpClient.Do(req)
			select {
			case c <- &httpPeek{newRes, newErr}:
			default:
				if newErr == nil && newRes != nil && newRes.Body != nil {
					// Discard and close so we can reuse it
					io.Copy(ioutil.Discard, newRes.Body)
					newRes.Body.Close()
				}
			}
			close(c)
		}()
		select {
		case <-time.After(r.Timeout):
		case v := <-c:
			if v != nil {
				res = v.res
				err = v.err
			}
		}
	} else {
		res, err = httpClient.Do(req)
	}
	if err != nil {
		return nil, err
	}
      // other code..
}
func init() {
	timeout := time.Minute
	httpClient = &http.Client{
		Transport: &http.Transport{
			MaxIdleConnsPerHost:   5,
			TLSClientConfig:       &tls.Config{InsecureSkipVerify: true},
			ResponseHeaderTimeout: timeout,
		},
		Timeout: timeout,
	}
}