diff --git a/jetty-websocket/websocket-core-server/src/main/java/org/eclipse/jetty/websocket/core/server/ServerUpgradeResponse.java b/jetty-websocket/websocket-core-server/src/main/java/org/eclipse/jetty/websocket/core/server/ServerUpgradeResponse.java
index e28b37fc61e..8c5b2450b5f 100644
--- a/jetty-websocket/websocket-core-server/src/main/java/org/eclipse/jetty/websocket/core/server/ServerUpgradeResponse.java
+++ b/jetty-websocket/websocket-core-server/src/main/java/org/eclipse/jetty/websocket/core/server/ServerUpgradeResponse.java
@@ -16,7 +16,6 @@ package org.eclipse.jetty.websocket.core.server;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -43,15 +42,14 @@ public class ServerUpgradeResponse
     {
         if (HttpHeader.SEC_WEBSOCKET_SUBPROTOCOL.is(name))
         {
-            setAcceptedSubProtocol(value); // Can only be one, so set
+            setAcceptedSubProtocol(value);
             return;
         }
 
-        if (HttpHeader.SEC_WEBSOCKET_EXTENSIONS.is(name) && getExtensions() != null)
+        if (HttpHeader.SEC_WEBSOCKET_EXTENSIONS.is(name))
         {
-            // Move any extensions configs to the headers
-            response.addHeader(name, ExtensionConfig.toHeaderValue(getExtensions()));
-            setExtensions(null);
+            addExtensions(ExtensionConfig.parseList(value));
+            return;
         }
 
         response.addHeader(name, value);
@@ -59,7 +57,6 @@ public class ServerUpgradeResponse
 
     public void setHeader(String name, String value)
     {
-
         if (HttpHeader.SEC_WEBSOCKET_SUBPROTOCOL.is(name))
         {
             setAcceptedSubProtocol(value);
@@ -67,7 +64,10 @@ public class ServerUpgradeResponse
         }
 
         if (HttpHeader.SEC_WEBSOCKET_EXTENSIONS.is(name))
-            setExtensions(null);
+        {
+            setExtensions(ExtensionConfig.parseList(value));
+            return;
+        }
 
         response.setHeader(name, value);
     }
@@ -86,14 +86,21 @@ public class ServerUpgradeResponse
 
         if (HttpHeader.SEC_WEBSOCKET_EXTENSIONS.is(name))
         {
-            setExtensions(null);
-            response.setHeader(name, null);
-            values.forEach(value -> addHeader(name, value));
+            List<ExtensionConfig> extensions = Collections.emptyList();
+            if (values != null)
+            {
+                extensions = values.stream()
+                    .flatMap(s -> ExtensionConfig.parseList(s).stream())
+                    .collect(Collectors.toList());
+            }
+
+            setExtensions(extensions);
             return;
         }
 
-        response.setHeader(name, null); // clear it out first
-        values.forEach(value -> response.addHeader(name, value));
+        response.setHeader(name, null);
+        if (values != null)
+            values.forEach(value -> response.addHeader(name, value));
     }
 
     public String getAcceptedSubProtocol()
@@ -113,7 +120,7 @@ public class ServerUpgradeResponse
 
     public Set<String> getHeaderNames()
     {
-        return Collections.unmodifiableSet(new HashSet<>(response.getHeaderNames()));
+        return Set.copyOf(response.getHeaderNames());
     }
 
     public Map<String, List<String>> getHeadersMap()
@@ -125,7 +132,7 @@ public class ServerUpgradeResponse
 
     public List<String> getHeaders(String name)
     {
-        return Collections.unmodifiableList(new ArrayList<>(response.getHeaders(name)));
+        return List.copyOf(response.getHeaders(name));
     }
 
     public int getStatusCode()
@@ -154,6 +161,14 @@ public class ServerUpgradeResponse
         negotiation.setSubprotocol(protocol);
     }
 
+    public void addExtensions(List<ExtensionConfig> configs)
+    {
+        ArrayList<ExtensionConfig> combinedConfig = new ArrayList<>();
+        combinedConfig.addAll(getExtensions());
+        combinedConfig.addAll(configs);
+        setExtensions(combinedConfig);
+    }
+
     public void setExtensions(List<ExtensionConfig> configs)
     {
         // This validation is also done later in RFC6455Handshaker but it is better to fail earlier
diff --git a/jetty-websocket/websocket-jetty-tests/src/test/java/org/eclipse/jetty/websocket/tests/JettyWebSocketNegotiationTest.java b/jetty-websocket/websocket-jetty-tests/src/test/java/org/eclipse/jetty/websocket/tests/JettyWebSocketNegotiationTest.java
index 9eeb08853dd..d4064afdbda 100644
--- a/jetty-websocket/websocket-jetty-tests/src/test/java/org/eclipse/jetty/websocket/tests/JettyWebSocketNegotiationTest.java
+++ b/jetty-websocket/websocket-jetty-tests/src/test/java/org/eclipse/jetty/websocket/tests/JettyWebSocketNegotiationTest.java
@@ -32,14 +32,13 @@ import org.eclipse.jetty.websocket.client.JettyUpgradeListener;
 import org.eclipse.jetty.websocket.client.WebSocketClient;
 import org.eclipse.jetty.websocket.server.JettyWebSocketServerContainer;
 import org.eclipse.jetty.websocket.server.config.JettyWebSocketServletContainerInitializer;
-import org.hamcrest.Matchers;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
-import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.is;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 public class JettyWebSocketNegotiationTest
@@ -112,7 +111,6 @@ public class JettyWebSocketNegotiationTest
         }
     }
 
-    @Disabled("Work in progress; ServerUpgradeResponse in websocket-core throws NPEs")
     @Test
     public void testManualNegotiationInCreator() throws Exception
     {
@@ -123,7 +121,7 @@ public class JettyWebSocketNegotiationTest
                 .filter(ec -> "permessage-deflate".equals(ec.getName()))
                 .filter(ec -> ec.getParameters().containsKey("client_no_context_takeover"))
                 .count();
-            assertThat(matchedExts, Matchers.is(1L));
+            assertThat(matchedExts, is(1L));
 
             // Manually drop the param so it is not negotiated in the extension stack.
             resp.setHeader("Sec-WebSocket-Extensions", "permessage-deflate");
@@ -146,6 +144,7 @@ public class JettyWebSocketNegotiationTest
 
         client.connect(socket, uri, upgradeRequest, upgradeListener).get(5, TimeUnit.SECONDS);
         HttpResponse httpResponse = responseReference.get();
-        System.err.println(httpResponse.getHeaders());
+        String extensions = httpResponse.getHeaders().get("Sec-WebSocket-Extensions");
+        assertThat(extensions, is("permessage-deflate"));
     }
 }