diff --git a/lib/__tests__/telemetry-state-read-error.test.ts b/lib/__tests__/telemetry-state-read-error.test.ts
index 8c11b79d0..aa630bdfd 100644
--- a/lib/__tests__/telemetry-state-read-error.test.ts
+++ b/lib/__tests__/telemetry-state-read-error.test.ts
@@ -40,6 +40,16 @@ describe('telemetry state that cannot be read', () => {
     expect((await effectiveConsent()).consent).toBe('on');
   });
 
+  it('refuses to write over it', async () => {
+    const { updateState } = await import('@/lib/telemetry/state');
+    mkdirSync(dir, { recursive: true });
+    const file = path.join(dir, 'state.json');
+    writeFileSync(file, '{"consent":"on","endp');
+
+    await expect(updateState((state) => { state.nextScheduledAt = 'x'; })).rejects.toThrow('could not be read');
+    expect(readFileSync(file, 'utf8')).toBe('{"consent":"on","endp');
+  });
+
   it('still writes the default state on a fresh install', async () => {
     const { loadState } = await import('@/lib/telemetry/state');
     expect((await loadState()).consent).toBe('off');
diff --git a/lib/telemetry/state.ts b/lib/telemetry/state.ts
index cde8620b6..9a82f2c2a 100644
--- a/lib/telemetry/state.ts
+++ b/lib/telemetry/state.ts
@@ -2,7 +2,7 @@ import path from 'node:path';
 import { randomUUID } from 'node:crypto';
 import { logger } from '@/lib/logger';
 import { getStore, type CollectionOptions } from '@/lib/shared-store';
-import { isStoreUnavailable } from '@/lib/stored-data-errors';
+import { isStoreUnavailable, StoredDataUnreadableError } from '@/lib/stored-data-errors';
 import type { TelemetryStateFile, ConsentState } from './types';
 import { DEFAULT_ENDPOINT } from './types';
 import { isNoThirdPartyTrafficOn } from '@/lib/admin/third-party-mode';
@@ -143,7 +143,8 @@ export async function saveState(state: TelemetryStateFile): Promise<void> {
 /**
  * Change some fields of the stored state without undoing a write that
  * landed meanwhile (another replica scheduling, the admin changing consent).
- * `change` may run more than once. Returns the state written.
+ * `change` may run more than once. Returns the state written. A state file
+ * that doesn't parse throws StoredDataUnreadableError and is left as it is.
  */
 export async function updateState(change: (state: TelemetryStateFile) => void): Promise<TelemetryStateFile> {
   let next: TelemetryStateFile = { ...DEFAULTS };
@@ -151,8 +152,10 @@ export async function updateState(change: (state: TelemetryStateFile) => void):
     let state: TelemetryStateFile;
     try {
       state = current ? parse(current) : { ...DEFAULTS };
-    } catch {
-      state = { ...DEFAULTS }; // a corrupt file: this write replaces it
+    } catch (err) {
+      // Unreadable is not missing, here as on reads: writing the defaults
+      // with one change would throw away the admin's consent and endpoint.
+      throw new StoredDataUnreadableError('telemetry state', { cause: err });
     }
     change(state);
     next = state;