diff --git a/configurator/src/components/ActiveApp.tsx b/configurator/src/components/ActiveApp.tsx
index 7c5d858b..4eccca4c 100644
--- a/configurator/src/components/ActiveApp.tsx
+++ b/configurator/src/components/ActiveApp.tsx
@@ -23,6 +23,8 @@ interface Props {
   layoutId: number;
   startChannel: number;
   params: Value[];
+  /** The app declares params but never answered a request for them. */
+  notResponding?: boolean;
 }
 
 // react-hook-form seeds each field's value once, when the form itself is
@@ -105,7 +107,13 @@ const ActiveAppParamsForm = ({
   );
 };
 
-export const ActiveApp = ({ app, layoutId, params, startChannel }: Props) => {
+export const ActiveApp = ({
+  app,
+  layoutId,
+  params,
+  startChannel,
+  notResponding,
+}: Props) => {
   return (
     <details className="group w-full bg-black">
       <summary
@@ -128,7 +136,17 @@ export const ActiveApp = ({ app, layoutId, params, startChannel }: Props) => {
         </div>
         <div className="flex-1">
           <p className="text-yellow-fp text-sm font-bold uppercase">App</p>
-          <p className="text-lg font-medium">{app.name}</p>
+          <p className="flex items-center gap-2 text-lg font-medium">
+            {app.name}
+            {notResponding && (
+              <span
+                className="rounded bg-red-900 px-2 py-0.5 text-xs font-bold tracking-wide text-red-200 uppercase"
+                title="This app declares parameters but hasn't answered a request for them — its params may be unreachable from the Configurator."
+              >
+                Not responding
+              </span>
+            )}
+          </p>
         </div>
         <div className="flex-1">
           <p className="text-yellow-fp text-sm font-bold uppercase">
@@ -164,7 +182,13 @@ export const ActiveApp = ({ app, layoutId, params, startChannel }: Props) => {
           <div className="w-7" />
         )}
       </summary>
-      {app.paramCount > 0 ? (
+      {app.paramCount > 0 && notResponding ? (
+        <div className="border-default-100 border-y-3 px-4 py-8 text-red-300">
+          This app hasn't answered a parameter request, so there's nothing real
+          to show here — showing empty or default values would be misleading,
+          and saving them could overwrite what the app actually holds.
+        </div>
+      ) : app.paramCount > 0 ? (
         <ActiveAppParamsForm
           key={JSON.stringify(params)}
           app={app}
diff --git a/configurator/src/components/ActiveApps.tsx b/configurator/src/components/ActiveApps.tsx
index 56c4c640..b2f55631 100644
--- a/configurator/src/components/ActiveApps.tsx
+++ b/configurator/src/components/ActiveApps.tsx
@@ -25,16 +25,19 @@ export const ActiveApps = () => {
           )
           .map(({ app, id, startChannel }) => {
             const params = allParams.get(id);
-            if (!params) {
-              return null;
-            }
+            // A missing entry is normal for an app with no params (it never
+            // gets asked). For one that declares params, it means the app
+            // never answered a param request — still worth a card, so the
+            // silence is visible instead of the app just not being listed.
+            const notResponding = app.paramCount > 0 && !params;
             return (
               <li key={id}>
                 <ActiveApp
                   app={app}
                   startChannel={startChannel}
                   layoutId={id}
-                  params={params}
+                  params={params ?? []}
+                  notResponding={notResponding}
                 />
               </li>
             );
diff --git a/faderpunk/src/tasks/configure.rs b/faderpunk/src/tasks/configure.rs
index ae47a849..dcf92c5e 100644
--- a/faderpunk/src/tasks/configure.rs
+++ b/faderpunk/src/tasks/configure.rs
@@ -246,6 +246,14 @@ pub async fn start_config_loop<'a>(usb_tx: &'a SharedUsbSender<'a>) {
                 proto.send_msg(ConfigMsgOut::GlobalConfig(config)).await
             }
             ConfigMsgIn::GetAppParams { layout_id } => {
+                // layout_id comes straight from the client with no validation
+                // elsewhere; every other caller indexes it from a real
+                // Layout, but this one goes over the wire raw. Answering
+                // nothing for an out-of-range id (same as an app that never
+                // replies) beats indexing APP_PARAM_SIGNALS out of bounds.
+                if layout_id as usize >= GLOBAL_CHANNELS {
+                    continue;
+                }
                 drain_app_param_responses();
                 APP_PARAM_SIGNALS[layout_id as usize].signal(AppParamCmd::RequestParamValues);
                 match receive_app_params(layout_id).await {
@@ -258,6 +266,9 @@ pub async fn start_config_loop<'a>(usb_tx: &'a SharedUsbSender<'a>) {
                 }
             }
             ConfigMsgIn::SetAppParams { layout_id, values } => {
+                if layout_id as usize >= GLOBAL_CHANNELS {
+                    continue;
+                }
                 drain_app_param_responses();
                 APP_PARAM_SIGNALS[layout_id as usize].signal(AppParamCmd::SetAppParams { values });
                 match receive_app_params(layout_id).await {