diff --git a/ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java b/ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java
index c6d41bad63..be118b8419 100644
--- a/ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java
+++ b/ambry-rest/src/main/java/com/github/ambry/rest/NettyResponseChannel.java
@@ -589,9 +589,12 @@ private FullHttpResponse getErrorResponse(Throwable cause) {
       errorResponseStatus = ResponseStatus.getResponseStatus(restServiceErrorCode);
       status = getHttpResponseStatus(errorResponseStatus);
       if (shouldSendFailureReason(status, restServiceException)) {
-        errReason = new String(
-            Utils.getRootCause(cause).getMessage().replaceAll("[\n\t\r]", " ").getBytes(StandardCharsets.US_ASCII),
-            StandardCharsets.US_ASCII);
+        String rootMessage = Utils.getRootCause(cause).getMessage();
+        if (rootMessage != null) {
+          errReason = new String(
+              rootMessage.replaceAll("[\n\t\r]", " ").getBytes(StandardCharsets.US_ASCII),
+              StandardCharsets.US_ASCII);
+        }
       }
       if (restServiceException.shouldIncludeExceptionMetadataInResponse()) {
         errHeaders = restServiceException.getExceptionHeadersMap();
diff --git a/ambry-rest/src/test/java/com/github/ambry/rest/NettyResponseChannelTest.java b/ambry-rest/src/test/java/com/github/ambry/rest/NettyResponseChannelTest.java
index 24c42699d1..a2fe47d958 100644
--- a/ambry-rest/src/test/java/com/github/ambry/rest/NettyResponseChannelTest.java
+++ b/ambry-rest/src/test/java/com/github/ambry/rest/NettyResponseChannelTest.java
@@ -462,6 +462,27 @@ public void setFailureReasonInResponseTest() throws Exception {
     assertFalse("Channel not closed on the server", channel.isActive());
   }
 
+  /**
+   * Tests that no NPE occurs when the root cause of a RestServiceException has a null message (e.g.,
+   * {@link java.util.concurrent.TimeoutException} created by {@link java.util.concurrent.CompletableFuture#orTimeout}).
+   */
+  @Test
+  public void setFailureReasonNullMessageNoNpeTest() throws Exception {
+    HttpRequest request =
+        createRequestWithHeaders(HttpMethod.GET, TestingUri.SetFailureReasonInResponseWithNullMessage.toString());
+    HttpUtil.setKeepAlive(request, false);
+
+    EmbeddedChannel channel = createEmbeddedChannel();
+    channel.writeInbound(request);
+
+    HttpResponse response = channel.readOutbound();
+    assertEquals(HttpResponseStatus.SERVICE_UNAVAILABLE, response.status());
+    // No FAILURE_REASON_HEADER when root cause message is null — no NPE should be thrown
+    assertFalse("Should not have failure reason header when root message is null",
+        response.headers().contains(NettyResponseChannel.FAILURE_REASON_HEADER));
+    assertFalse("Channel not closed on the server", channel.isActive());
+  }
+
   /**
    * Sends null input to {@link NettyResponseChannel#setHeader(String, Object)} (through
    * {@link MockNettyMessageProcessor}) and tests for reaction.
@@ -1339,6 +1360,11 @@ enum TestingUri {
    * to the client.
    */
   SetFailureReasonInResponseWithException,
+  /**
+   * When this request is received, a RestServiceException whose root cause has a null message is used on
+   * onResponseComplete. Verifies no NPE when the root cause message is null.
+   */
+  SetFailureReasonInResponseWithNullMessage,
   /**
    * Catch all TestingUri.
    */
@@ -1584,6 +1610,12 @@ private void handleRequest(HttpRequest httpRequest) throws Exception {
         request.setArg(RestUtils.InternalKeys.SEND_FAILURE_REASON, Boolean.TRUE);
         restResponseChannel.onResponseComplete(new Exception(TestingUri.SetFailureReasonInResponse.toString()));
         break;
+      case SetFailureReasonInResponseWithNullMessage:
+        request.setArg(RestUtils.InternalKeys.SEND_FAILURE_REASON, Boolean.TRUE);
+        // TimeoutException() with no message simulates CompletableFuture.orTimeout() behavior
+        restResponseChannel.onResponseComplete(new RestServiceException("outer message",
+            new TimeoutException(), RestServiceErrorCode.ServiceUnavailable));
+        break;
     }
   }