diff --git a/jetty-proxy/src/main/java/org/eclipse/jetty/proxy/ProxyServlet.java b/jetty-proxy/src/main/java/org/eclipse/jetty/proxy/ProxyServlet.java
index 5b2fa5676e7..e37bd297064 100644
--- a/jetty-proxy/src/main/java/org/eclipse/jetty/proxy/ProxyServlet.java
+++ b/jetty-proxy/src/main/java/org/eclipse/jetty/proxy/ProxyServlet.java
@@ -534,6 +534,10 @@ public class ProxyServlet extends HttpServlet
 
     protected void onResponseHeaders(HttpServletRequest request, HttpServletResponse response, Response proxyResponse)
     {
+        // Clear the response headers in case it comes with predefined ones.
+        for (String name : response.getHeaderNames())
+            response.setHeader(name, null);
+
         for (HttpField field : proxyResponse.getHeaders())
         {
             String headerName = field.getName();
@@ -545,7 +549,7 @@ public class ProxyServlet extends HttpServlet
             if (newHeaderValue == null || newHeaderValue.trim().length() == 0)
                 continue;
 
-            response.setHeader(headerName, newHeaderValue);
+            response.addHeader(headerName, newHeaderValue);
         }
     }