diff -r 84a8fa684998 -r 5c29612f27b4 src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java
--- a/src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java	Mon Apr 09 12:59:06 2018 +0100
+++ b/src/java.net.http/share/classes/jdk/internal/net/http/Http2Connection.java	Mon Apr 09 15:10:48 2018 +0100
@@ -36,6 +36,7 @@
 import java.util.Iterator;
 import java.util.LinkedList;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.CompletableFuture;
@@ -426,7 +427,7 @@
 
         return keyString(isSecure, isProxy, addr.getHostString(), addr.getPort());
     }
-    
+
     static String keyFor(URI uri, InetSocketAddress proxy) {
         boolean isSecure = uri.getScheme().equalsIgnoreCase("https");
         boolean isProxy = proxy != null;
@@ -960,7 +961,7 @@
         List<ByteBuffer> buffers = new ArrayList<>();
         for(HttpHeaders header : headers) {
             for (Map.Entry<String, List<String>> e : header.map().entrySet()) {
-                String lKey = e.getKey().toLowerCase();
+                String lKey = e.getKey().toLowerCase(Locale.US);
                 List<String> values = e.getValue();
                 for (String value : values) {
                     hpackOut.header(lKey, value);