diff --git a/apps/api/src/routes/admin.ts b/apps/api/src/routes/admin.ts
index a494231..4c0c34c 100644
--- a/apps/api/src/routes/admin.ts
+++ b/apps/api/src/routes/admin.ts
@@ -222,10 +222,14 @@ adminRouter.post('/members', zValidator('json', createMemberSchema, noEchoHook),
 //   - parsePositiveIntParam: rejects malformed ids (T-20-03)
 // ---------------------------------------------------------------------------
 
-const updateMemberSchema = z.object({
-  displayName: z.string().min(1).max(256).optional(),
-  isAdmin: z.boolean().optional(),
-});
+const updateMemberSchema = z
+  .object({
+    displayName: z.string().min(1).max(256).optional(),
+    isAdmin: z.boolean().optional(),
+  })
+  .refine((data) => data.displayName !== undefined || data.isAdmin !== undefined, {
+    message: 'At least one field must be provided',
+  });
 
 adminRouter.patch('/members/:id', zValidator('json', updateMemberSchema, noEchoHook), async (c) => {
   const targetId = parsePositiveIntParam(c.req.param('id'));
@@ -236,35 +240,58 @@ adminRouter.patch('/members/:id', zValidator('json', updateMemberSchema, noEchoH
   const { displayName, isAdmin } = c.req.valid('json');
   // T-20-04: NEVER log request body
 
-  // Verify the target user exists (404 if not)
-  const [target] = await db
-    .select({ id: users.id, isAdmin: users.isAdmin })
-    .from(users)
-    .where(eq(users.id, targetId))
-    .limit(1);
-
-  if (!target) {
-    return c.json({ error: 'Member not found' }, 404);
-  }
-
-  // D-03 last-admin guard: reject demotion of the only remaining admin (T-20-02)
-  if (isAdmin === false && target.isAdmin) {
-    const [{ count }] = await db
-      .select({ count: sql<number>`COUNT(*)` })
-      .from(users)
-      .where(eq(users.isAdmin, true));
-    if (Number(count) <= 1) {
-      return c.json({ error: 'Cannot remove the last admin' }, 409);
-    }
-  }
-
-  // Build a partial set() from whichever fields are present
+  // Build a partial set() from whichever fields are present — validated non-empty by schema
   const updates: { displayName?: string; isAdmin?: boolean } = {};
   if (displayName !== undefined) updates.displayName = displayName;
   if (isAdmin !== undefined) updates.isAdmin = isAdmin;
 
   try {
-    await db.update(users).set(updates).where(eq(users.id, targetId));
+    // CR-01: wrap the guard + update in a transaction so the last-admin check and
+    // the UPDATE are atomic. Without a transaction, two concurrent demotions both
+    // read count=2, both pass the guard, and both commit — leaving zero admins.
+    // The locking read (FOR UPDATE via raw SQL suffix) serialises concurrent
+    // demotions: the second PATCH blocks until the first commits and then re-reads
+    // a count of 1, triggering the 'last-admin' error correctly.
+    let lastAdminViolation = false;
+    let notFound = false;
+
+    await db.transaction(async (tx) => {
+      // Re-read the target inside the transaction so we see the committed state
+      const [target] = await tx
+        .select({ id: users.id, isAdmin: users.isAdmin })
+        .from(users)
+        .where(eq(users.id, targetId))
+        .limit(1);
+
+      if (!target) {
+        notFound = true;
+        return;
+      }
+
+      // D-03 last-admin guard: reject demotion of the only remaining admin (T-20-02).
+      // Use a raw locking read to serialise concurrent demotions. Drizzle 0.45.x does
+      // not expose a first-class .for('update') on select; appending FOR UPDATE via a
+      // raw sql suffix achieves the same serialisation in InnoDB.
+      if (isAdmin === false && target.isAdmin) {
+        const [[{ count }]] = (await tx.execute(
+          sql`SELECT COUNT(*) AS count FROM ${users} WHERE ${users.isAdmin} = true FOR UPDATE`,
+        )) as unknown as [{ count: number | string }[], unknown];
+        if (Number(count) <= 1) {
+          lastAdminViolation = true;
+          return;
+        }
+      }
+
+      await tx.update(users).set(updates).where(eq(users.id, targetId));
+    });
+
+    if (notFound) {
+      return c.json({ error: 'Member not found' }, 404);
+    }
+    if (lastAdminViolation) {
+      return c.json({ error: 'Cannot remove the last admin' }, 409);
+    }
+
     return c.json({ ok: true }, 200);
   } catch (err) {
     console.error(
diff --git a/apps/api/tests/routes/admin.test.ts b/apps/api/tests/routes/admin.test.ts
index 62aa3d0..21bc7ee 100644
--- a/apps/api/tests/routes/admin.test.ts
+++ b/apps/api/tests/routes/admin.test.ts
@@ -1226,6 +1226,21 @@ describe('PATCH /api/admin/members/:id', () => {
     );
     expect(res.status).toBe(404);
   });
+
+  // Test H (WR-06): empty {} body must return 400, not crash Drizzle with a 503
+  it('Test H (WR-06 empty body): PATCH with {} returns 400 { error: "Invalid request" }', async () => {
+    const adminId = await seedUser('admin-patch-empty', true);
+    const memberId = await seedUser('member-patch-empty', false);
+    currentDevUserId = adminId;
+    const app = await getApp();
+
+    const res = await app.fetch(
+      jsonRequest('PATCH', `/api/admin/members/${memberId}`, {}),
+    );
+    expect(res.status).toBe(400);
+    const body = (await res.json()) as { error: string };
+    expect(body.error).toBe('Invalid request');
+  });
 });
 
 // ===========================================================================