diff --git a/h2quic/client.go b/h2quic/client.go
index d34863eb..58534711 100644
--- a/h2quic/client.go
+++ b/h2quic/client.go
@@ -158,9 +158,10 @@ func (c *Client) Do(req *http.Request) (*http.Response, error) {
 	for c.encryptionLevel != protocol.EncryptionForwardSecure {
 		c.cryptoChangedCond.Wait()
 	}
-
 	hdrChan := make(chan *http.Response)
 	c.responses[dataStreamID] = hdrChan
+	c.mutex.Unlock()
+
 	dataStream, err := c.client.OpenStream(dataStreamID)
 	if err != nil {
 		return nil, err
@@ -169,7 +170,6 @@ func (c *Client) Do(req *http.Request) (*http.Response, error) {
 	if err != nil {
 		return nil, err
 	}
-	c.mutex.Unlock()
 
 	var res *http.Response
 	select {
diff --git a/h2quic/request_writer.go b/h2quic/request_writer.go
index c77fcbd6..7f421adc 100644
--- a/h2quic/request_writer.go
+++ b/h2quic/request_writer.go
@@ -6,6 +6,7 @@ import (
 	"net/http"
 	"strconv"
 	"strings"
+	"sync"
 
 	"golang.org/x/net/http2"
 	"golang.org/x/net/http2/hpack"
@@ -16,6 +17,7 @@ import (
 )
 
 type requestWriter struct {
+	mutex        sync.Mutex
 	headerStream utils.Stream
 
 	henc *hpack.Encoder
@@ -36,6 +38,10 @@ func (w *requestWriter) WriteRequest(req *http.Request, dataStreamID protocol.St
 	// TODO: add support for trailers
 	// TODO: add support for gzip compression
 	// TODO: write continuation frames, if the header frame is too long
+
+	w.mutex.Lock()
+	defer w.mutex.Unlock()
+
 	w.encodeHeaders(req, false, "", actualContentLength(req))
 	h2framer := http2.NewFramer(w.headerStream, nil)
 	return h2framer.WriteHeaders(http2.HeadersFrameParam{