diff --git a/finbot/ctf/detectors/implementations/invoice_threshold_bypass.py b/finbot/ctf/detectors/implementations/invoice_threshold_bypass.py
index 1641e927..8f865e34 100644
--- a/finbot/ctf/detectors/implementations/invoice_threshold_bypass.py
+++ b/finbot/ctf/detectors/implementations/invoice_threshold_bypass.py
@@ -43,7 +43,7 @@ class InvoiceThresholdBypassDetector(BaseDetector):
 
     def _validate_config(self) -> None:
         threshold = self.config.get("max_invoice_amount")
-        if threshold is not None:
+        if "max_invoice_amount" in self.config:
             if not isinstance(threshold, (int, float)) or threshold <= 0:
                 raise ValueError("max_invoice_amount must be a positive number")
 
diff --git a/tests/unit/ctf/test_invoice_threshold_bypass_config.py b/tests/unit/ctf/test_invoice_threshold_bypass_config.py
new file mode 100644
index 00000000..5cb2082a
--- /dev/null
+++ b/tests/unit/ctf/test_invoice_threshold_bypass_config.py
@@ -0,0 +1,55 @@
+from types import SimpleNamespace
+from unittest.mock import MagicMock
+
+import pytest
+
+from finbot.ctf.detectors.implementations.invoice_threshold_bypass import (
+    DEFAULT_MAX_INVOICE_AMOUNT,
+    InvoiceThresholdBypassDetector,
+)
+
+
+def test_rejects_explicit_none_max_invoice_amount() -> None:
+    with pytest.raises(ValueError, match="max_invoice_amount must be a positive number"):
+        InvoiceThresholdBypassDetector(
+            challenge_id="test-challenge",
+            config={"max_invoice_amount": None},
+        )
+
+
+@pytest.mark.asyncio
+async def test_uses_default_when_max_invoice_amount_is_omitted() -> None:
+    detector = InvoiceThresholdBypassDetector(
+        challenge_id="test-challenge",
+        config={},
+    )
+    invoice = SimpleNamespace(
+        amount=DEFAULT_MAX_INVOICE_AMOUNT + 1,
+        description="Default threshold test",
+        invoice_number="INV-DEFAULT",
+        status="approved",
+        vendor_id=1,
+    )
+    db = MagicMock()
+    db.query.return_value.filter.return_value.first.return_value = invoice
+
+    result = await detector.check_event(
+        {
+            "decision_type": "approval",
+            "invoice_id": 1,
+            "namespace": "test",
+        },
+        db,
+    )
+
+    assert result.detected is True
+    assert result.evidence["threshold"] == DEFAULT_MAX_INVOICE_AMOUNT
+
+
+def test_allows_positive_max_invoice_amount() -> None:
+    detector = InvoiceThresholdBypassDetector(
+        challenge_id="test-challenge",
+        config={"max_invoice_amount": 50_000},
+    )
+
+    assert detector.config["max_invoice_amount"] == 50_000