diff --git a/electron/main/process-instance-lock.ts b/electron/main/process-instance-lock.ts
index 154d36e..72bdc19 100644
--- a/electron/main/process-instance-lock.ts
+++ b/electron/main/process-instance-lock.ts
@@ -76,7 +76,9 @@ export function acquireProcessInstanceFileLock(
       }
 
       ownerPid = readLockOwnerPid(lockPath);
-      if (ownerPid && !isPidAlive(ownerPid) && existsSync(lockPath)) {
+      const shouldTreatAsStale =
+        ownerPid === undefined || !isPidAlive(ownerPid);
+      if (shouldTreatAsStale && existsSync(lockPath)) {
         try {
           rmSync(lockPath, { force: true });
           continue;
diff --git a/tests/unit/process-instance-lock.test.ts b/tests/unit/process-instance-lock.test.ts
index ebf59c4..dadb579 100644
--- a/tests/unit/process-instance-lock.test.ts
+++ b/tests/unit/process-instance-lock.test.ts
@@ -77,4 +77,20 @@ describe('process instance file lock', () => {
     expect(readFileSync(lockPath, 'utf8')).toBe('5555');
     lock.release();
   });
+
+  it('replaces malformed lock file content', () => {
+    const userDataDir = createTempDir();
+    const lockPath = join(userDataDir, 'clawx.instance.lock');
+    writeFileSync(lockPath, 'not-a-pid', 'utf8');
+
+    const lock = acquireProcessInstanceFileLock({
+      userDataDir,
+      lockName: 'clawx',
+      pid: 6666,
+    });
+
+    expect(lock.acquired).toBe(true);
+    expect(readFileSync(lockPath, 'utf8')).toBe('6666');
+    lock.release();
+  });
 });