diff --git a/src/lib/sharing/grants.ts b/src/lib/sharing/grants.ts
index 38d9f71e5..183e38a55 100644
--- a/src/lib/sharing/grants.ts
+++ b/src/lib/sharing/grants.ts
@@ -983,7 +983,20 @@ async function endGrant(
     (current.expiresAt === null || current.expiresAt > new Date());
 
   if (!actorOwnsGrant || !isActiveManagedGuardian) return end();
-  return reduceManagedProfileGuardian(db, current.grantorId, end);
+  // `current` was read before the record lock. A concurrent change that holds
+  // the lock first (the Guardian's account deletion ends this grant) commits
+  // in between, and counting under the lock would then refuse with "add
+  // another Guardian" for a grant that is no longer active, instead of saying
+  // it has already ended. So the grant is re-read under the lock; only a grant
+  // still live there is a Guardian whose removal the count has to allow.
+  return withManagedProfileLock(db, current.grantorId, async () => {
+    const live = await db.accountGrant.findFirst({
+      where: { id: grantId, revokedAt: null },
+      select: { id: true },
+    });
+    if (!live) return end();
+    return reduceManagedProfileGuardian(db, current.grantorId, end);
+  });
 }
 
 /**
diff --git a/tests/integration/managed-profile-lifecycle.test.ts b/tests/integration/managed-profile-lifecycle.test.ts
index dbb4bc92e..bd570ee3b 100644
--- a/tests/integration/managed-profile-lifecycle.test.ts
+++ b/tests/integration/managed-profile-lifecycle.test.ts
@@ -771,7 +771,18 @@ describe("managed profile lifecycle (real Postgres)", () => {
       ),
     ]);
 
-    expect([200, 404]).toContain(renounced.status);
+    // Either order is a correct serialization: the renunciation lands first
+    // (200), or the deletion ends the grant first and the renunciation finds
+    // it gone (404) or already ended (409 `sharing.renounce.already_ended`).
+    // Which one wins depends only on timing, and the write ceiling charged
+    // ahead of the renunciation shifts that timing.
+    expect([200, 404, 409]).toContain(renounced.status);
+    if (renounced.status === 409) {
+      const body = (await renounced.clone().json()) as {
+        meta?: { errorCode?: string };
+      };
+      expect(body.meta?.errorCode).toBe("sharing.renounce.already_ended");
+    }
     expect(deleted.status).toBe(200);
     await expect(
       Promise.all([
@@ -818,7 +829,18 @@ describe("managed profile lifecycle (real Postgres)", () => {
       ),
     ]);
 
-    expect([200, 404]).toContain(renounced.status);
+    // Either order is a correct serialization: the renunciation lands first
+    // (200), or the deletion ends the grant first and the renunciation finds
+    // it gone (404) or already ended (409 `sharing.renounce.already_ended`).
+    // Which one wins depends only on timing, and the write ceiling charged
+    // ahead of the renunciation shifts that timing.
+    expect([200, 404, 409]).toContain(renounced.status);
+    if (renounced.status === 409) {
+      const body = (await renounced.clone().json()) as {
+        meta?: { errorCode?: string };
+      };
+      expect(body.meta?.errorCode).toBe("sharing.renounce.already_ended");
+    }
     expect(deleted.status).toBe(200);
     await expect(
       Promise.all([