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/finbot/ctf/detectors/implementations/invoice_trust_override.py b/finbot/ctf/detectors/implementations/invoice_trust_override.py
index 26cb3db0..748a1e87 100644
--- a/finbot/ctf/detectors/implementations/invoice_trust_override.py
+++ b/finbot/ctf/detectors/implementations/invoice_trust_override.py
@@ -46,7 +46,7 @@ class InvoiceTrustOverrideDetector(BaseDetector):
 
     def _validate_config(self) -> None:
         min_amount = self.config.get("min_amount")
-        if min_amount is not None:
+        if "min_amount" in self.config:
             if not isinstance(min_amount, (int, float)) or min_amount <= 0:
                 raise ValueError("min_amount must be a positive number")
 
diff --git a/tests/unit/ctf/test_detector_numeric_config_validation.py b/tests/unit/ctf/test_detector_numeric_config_validation.py
new file mode 100644
index 00000000..1f661737
--- /dev/null
+++ b/tests/unit/ctf/test_detector_numeric_config_validation.py
@@ -0,0 +1,67 @@
+"""
+Numeric detector configuration validation tests.
+
+Covers the numeric detector config keys that are validated in
+_validate_config() and then read back with a default in check_event().
+A key present with a None value has to be rejected at construction,
+otherwise the default never applies and check_event() compares against
+None, which raises TypeError and takes the detector out for the rest of
+the process.
+
+This is the numeric counterpart of the list-typed keys covered in
+test_detector_config_validation.py.
+"""
+
+import pytest
+
+from finbot.ctf.detectors.implementations.invoice_threshold_bypass import (
+    DEFAULT_MAX_INVOICE_AMOUNT,
+    InvoiceThresholdBypassDetector,
+)
+from finbot.ctf.detectors.implementations.invoice_trust_override import (
+    DEFAULT_MIN_AMOUNT,
+    InvoiceTrustOverrideDetector,
+)
+
+NUMERIC_CONFIG_KEYS = [
+    (InvoiceThresholdBypassDetector, "max_invoice_amount", DEFAULT_MAX_INVOICE_AMOUNT),
+    (InvoiceTrustOverrideDetector, "min_amount", DEFAULT_MIN_AMOUNT),
+]
+
+
+class TestNoneRejected:
+    """A present-but-None numeric config must not reach check_event()."""
+
+    @pytest.mark.parametrize("detector_cls, key, _default", NUMERIC_CONFIG_KEYS)
+    def test_none_value_rejected(self, detector_cls, key, _default) -> None:
+        with pytest.raises(ValueError, match="positive number"):
+            detector_cls(challenge_id="c", config={key: None})
+
+    @pytest.mark.parametrize("detector_cls, key, _default", NUMERIC_CONFIG_KEYS)
+    def test_non_numeric_still_rejected(self, detector_cls, key, _default) -> None:
+        with pytest.raises(ValueError, match="positive number"):
+            detector_cls(challenge_id="c", config={key: "not a number"})
+
+    @pytest.mark.parametrize("detector_cls, key, _default", NUMERIC_CONFIG_KEYS)
+    def test_non_positive_still_rejected(self, detector_cls, key, _default) -> None:
+        with pytest.raises(ValueError, match="positive number"):
+            detector_cls(challenge_id="c", config={key: 0})
+
+
+class TestValidConfigs:
+    """The accepted shapes keep working."""
+
+    @pytest.mark.parametrize("detector_cls, key, _default", NUMERIC_CONFIG_KEYS)
+    def test_positive_value_accepted(self, detector_cls, key, _default) -> None:
+        detector = detector_cls(challenge_id="c", config={key: 5000})
+        assert detector.config[key] == 5000
+
+    @pytest.mark.parametrize("detector_cls, key, default", NUMERIC_CONFIG_KEYS)
+    def test_omitted_key_falls_back_to_default(self, detector_cls, key, default) -> None:
+        detector = detector_cls(challenge_id="c", config={})
+        assert detector.config.get(key, default) == default
+
+    @pytest.mark.parametrize("detector_cls, key, default", NUMERIC_CONFIG_KEYS)
+    def test_no_config_falls_back_to_default(self, detector_cls, key, default) -> None:
+        detector = detector_cls(challenge_id="c")
+        assert detector.config.get(key, default) == default