diff --git a/core/src/main/java/io/undertow/client/spdy/SpdyClientConnection.java b/core/src/main/java/io/undertow/client/spdy/SpdyClientConnection.java
index 2625a47167..979ff0284b 100644
--- a/core/src/main/java/io/undertow/client/spdy/SpdyClientConnection.java
+++ b/core/src/main/java/io/undertow/client/spdy/SpdyClientConnection.java
@@ -85,7 +85,7 @@ public void handleEvent(SpdyChannel channel) {
     }
 
     @Override
-    public void sendRequest(ClientRequest request, ClientCallback<ClientExchange> clientCallback) {
+    public synchronized void sendRequest(ClientRequest request, ClientCallback<ClientExchange> clientCallback) {
         request.getRequestHeaders().put(PATH, request.getPath());
         request.getRequestHeaders().put(SCHEME, "https");
         request.getRequestHeaders().put(VERSION, request.getProtocol().toString());