diff --git a/lnbits/core/services/users.py b/lnbits/core/services/users.py
index cd0b4ea06..37f2c998c 100644
--- a/lnbits/core/services/users.py
+++ b/lnbits/core/services/users.py
@@ -199,8 +199,11 @@ async def init_admin_settings(super_user: str | None = None) -> SuperSettings:
 async def check_register_activation_settings(data: RegisterUser):
     if not settings.lnbits_require_user_activation:
         return None
-    code = data.invitation_code.strip() if data.invitation_code else None
-    if settings.lnbits_user_activation_by_invitation_code and code:
+    if settings.lnbits_user_activation_by_invitation_code:
+        code = data.invitation_code.strip() if data.invitation_code else ""
+        if len(code) == 0:
+            raise ValueError("Invitation code cannot be empty.")
+
         if code == settings.lnbits_register_reusable_activation_code:
             return None
         if code in settings.lnbits_register_one_time_activation_codes:
diff --git a/tests/api/test_auth.py b/tests/api/test_auth.py
index 4504ffd78..cf50e575a 100644
--- a/tests/api/test_auth.py
+++ b/tests/api/test_auth.py
@@ -297,6 +297,42 @@ async def test_register_ok(http_client: AsyncClient):
     ), f"Expected 1 default wallet, not {len(user.wallets)}."
 
 
+@pytest.mark.anyio
+async def test_register_no_activation_code(
+    http_client: AsyncClient, settings: Settings
+):
+    settings.lnbits_require_user_activation = True
+
+    tiny_id = shortuuid.uuid()[:8]
+    response = await http_client.post(
+        "/api/v1/auth/register",
+        json={
+            "username": f"u21.{tiny_id}",
+            "password": "secret1234",
+            "password_repeat": "secret1234",
+            "email": f"u21.{tiny_id}@lnbits.com",
+        },
+    )
+
+    assert response.status_code == 400
+    assert response.json().get("detail") == "No activation method provided."
+
+    settings.lnbits_user_activation_by_invitation_code = True
+
+    response = await http_client.post(
+        "/api/v1/auth/register",
+        json={
+            "username": f"u21.{tiny_id}",
+            "password": "secret1234",
+            "password_repeat": "secret1234",
+            "email": f"u21.{tiny_id}@lnbits.com",
+        },
+    )
+
+    assert response.status_code == 400, "User creation blocked without activation code."
+    assert response.json().get("detail") == "Invitation code cannot be empty."
+
+
 @pytest.mark.anyio
 async def test_register_email_twice(http_client: AsyncClient):
     tiny_id = shortuuid.uuid()[:8]
diff --git a/tests/conftest.py b/tests/conftest.py
index a76168a38..a677a225b 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -341,3 +341,5 @@ def _settings_cleanup(settings: Settings):
     settings.lnbits_max_outgoing_payment_amount_sats = 10_000_000_100
     settings.lnbits_max_incoming_payment_amount_sats = 10_000_000_200
     settings.stripe_limits = FiatProviderLimits()
+    settings.lnbits_require_user_activation = False
+    settings.lnbits_user_activation_by_invitation_code = False