diff --git a/src/collaboration/awareness.test.ts b/src/collaboration/awareness.test.ts
index 4d41fbca..bbaa6e98 100644
--- a/src/collaboration/awareness.test.ts
+++ b/src/collaboration/awareness.test.ts
@@ -218,6 +218,14 @@ describe('collaboration awareness presentation', () => {
     expect(collaborationConnectionLabel(status)).toBe(label);
   });
 
+  it('rejects a runtime connection status outside the public states', () => {
+    expect(() => collaborationConnectionLabel('failed' as never)).toThrowError(
+      new RangeError(
+        'Collaboration connection status must be connecting, connected, disconnected, or offline.',
+      ),
+    );
+  });
+
   it('renders a text-only high-contrast cursor for valid remote data', () => {
     const cursor = renderCollaborationCursor({
       name: 'Remote Alice',
diff --git a/src/collaboration/awareness.ts b/src/collaboration/awareness.ts
index 6525b580..149b8028 100644
--- a/src/collaboration/awareness.ts
+++ b/src/collaboration/awareness.ts
@@ -215,6 +215,8 @@ export function collaborationConnectionLabel(
   status: CollaborationConnectionStatus | undefined,
 ): string {
   switch (status) {
+    case undefined:
+      return 'Collaboration ready';
     case 'connecting':
       return 'Connecting';
     case 'connected':
@@ -224,7 +226,9 @@ export function collaborationConnectionLabel(
     case 'offline':
       return 'Offline';
     default:
-      return 'Collaboration ready';
+      throw new RangeError(
+        'Collaboration connection status must be connecting, connected, disconnected, or offline.',
+      );
   }
 }