diff --git a/frontend-modern/src/components/Settings/AIModelSelectionSection.tsx b/frontend-modern/src/components/Settings/AIModelSelectionSection.tsx
index 3998a85e4..7ce7e46e1 100644
--- a/frontend-modern/src/components/Settings/AIModelSelectionSection.tsx
+++ b/frontend-modern/src/components/Settings/AIModelSelectionSection.tsx
@@ -126,7 +126,8 @@ export const PatrolPreflightControl: Component<{ state: AISettingsState }> = (co
   };
 
   const formatDuration = (ms: number) => {
-    if (ms < 1000) return `${ms}ms`;
+    if (!Number.isFinite(ms) || ms < 0) return '-';
+    if (ms < 1000) return `${Math.round(ms)}ms`;
     return `${(ms / 1000).toFixed(1)}s`;
   };
 
diff --git a/frontend-modern/src/features/alerts/useAlertScheduleState.ts b/frontend-modern/src/features/alerts/useAlertScheduleState.ts
index be1be54a3..28297c3fa 100644
--- a/frontend-modern/src/features/alerts/useAlertScheduleState.ts
+++ b/frontend-modern/src/features/alerts/useAlertScheduleState.ts
@@ -254,9 +254,10 @@ export function useAlertScheduleState(props: UseAlertScheduleStateProps) {
   };
 
   const setGroupingWindow = (rawValue: string) => {
+    const parsed = Number.parseInt(rawValue, 10);
     props.setGrouping({
       ...props.grouping(),
-      window: Number.parseInt(rawValue, 10),
+      window: Number.isNaN(parsed) ? props.grouping().window : parsed,
     });
     markUnsaved();
   };