diff --git a/app/api/instances/sync/route.ts b/app/api/instances/sync/route.ts
index a2e62313..7861d59b 100644
--- a/app/api/instances/sync/route.ts
+++ b/app/api/instances/sync/route.ts
@@ -3,7 +3,11 @@ import { timingSafeEqual } from "crypto";
 import { applyCaddyConfig } from "@/src/lib/caddy";
 import { applySyncPayload, getInstanceMode, getSlaveMasterToken, setSlaveLastSync, SyncPayload } from "@/src/lib/instance-sync";
 
-const MAX_SYNC_BODY_BYTES = Number(process.env.INSTANCE_SYNC_MAX_BYTES ?? 10 * 1024 * 1024);
+const DEFAULT_MAX_SYNC_BODY_BYTES = 10 * 1024 * 1024; // 10 MB
+const _parsedMaxBytes = Number(process.env.INSTANCE_SYNC_MAX_BYTES);
+const MAX_SYNC_BODY_BYTES = Number.isFinite(_parsedMaxBytes) && _parsedMaxBytes > 0
+  ? _parsedMaxBytes
+  : DEFAULT_MAX_SYNC_BODY_BYTES;
 const SYNC_RATE_MAX = Number(process.env.INSTANCE_SYNC_RATE_MAX ?? 60);
 const SYNC_RATE_WINDOW_MS = Number(process.env.INSTANCE_SYNC_RATE_WINDOW_MS ?? 60_000);
 const SYNC_RATE_LIMITS = new Map<string, { count: number; windowStart: number }>();
@@ -235,7 +239,7 @@ export async function POST(request: NextRequest) {
     return NextResponse.json({ ok: true });
   } catch (error) {
     const message = error instanceof Error ? error.message : "Failed to apply sync payload";
-    await setSlaveLastSync({ ok: false, error: message });
-    return NextResponse.json({ error: message }, { status: 500 });
+    await setSlaveLastSync({ ok: false, error: message }); // still store internally
+    return NextResponse.json({ error: "Failed to apply sync payload" }, { status: 500 });
   }
 }
diff --git a/drizzle/0008_unique_provider_subject.sql b/drizzle/0008_unique_provider_subject.sql
new file mode 100644
index 00000000..9130645f
--- /dev/null
+++ b/drizzle/0008_unique_provider_subject.sql
@@ -0,0 +1,2 @@
+DROP INDEX IF EXISTS `users_provider_subject_idx`;
+CREATE UNIQUE INDEX `users_provider_subject_idx` ON `users`(`provider`,`subject`);
diff --git a/drizzle/meta/_journal.json b/drizzle/meta/_journal.json
index caa68170..059f3376 100644
--- a/drizzle/meta/_journal.json
+++ b/drizzle/meta/_journal.json
@@ -57,6 +57,13 @@
       "when": 1740441600000,
       "tag": "0007_linking_tokens",
       "breakpoints": true
+    },
+    {
+      "idx": 8,
+      "version": "6",
+      "when": 1740960000000,
+      "tag": "0008_unique_provider_subject",
+      "breakpoints": true
     }
   ]
 }
diff --git a/src/lib/db/schema.ts b/src/lib/db/schema.ts
index e9400a0e..5a98f78b 100644
--- a/src/lib/db/schema.ts
+++ b/src/lib/db/schema.ts
@@ -17,7 +17,7 @@ export const users = sqliteTable(
   },
   (table) => ({
     emailUnique: uniqueIndex("users_email_unique").on(table.email),
-    providerSubjectIdx: index("users_provider_subject_idx").on(table.provider, table.subject)
+    providerSubjectIdx: uniqueIndex("users_provider_subject_idx").on(table.provider, table.subject)
   })
 );