diff --git a/src/anomalyDetector.ts b/src/anomalyDetector.ts
index 7f69bbb..3df8f5b 100644
--- a/src/anomalyDetector.ts
+++ b/src/anomalyDetector.ts
@@ -77,7 +77,11 @@ export class AnomalyDetector {
     this.rapidCycleSeconds = options.rapidCycleSeconds ?? 300;
     this.maxAmountVariance = options.maxAmountVariance ?? 0.8;
     this.sensitivityThreshold = options.sensitivityThreshold ?? 0.8;
-    if (this.sensitivityThreshold <= 0 || this.sensitivityThreshold > 1) {
+    if (
+      !Number.isFinite(this.sensitivityThreshold) ||
+      this.sensitivityThreshold <= 0 ||
+      this.sensitivityThreshold > 1
+    ) {
       throw new RangeError("sensitivityThreshold must be in the range (0, 1]");
     }
     this.now = options.now ?? (() => Math.floor(Date.now() / 1000));
diff --git a/test/anomalyDetector.nan.test.ts b/test/anomalyDetector.nan.test.ts
new file mode 100644
index 0000000..7378153
--- /dev/null
+++ b/test/anomalyDetector.nan.test.ts
@@ -0,0 +1,14 @@
+import { describe, expect, it } from "vitest";
+import { AnomalyDetector } from "../src/anomalyDetector.js";
+
+describe("AnomalyDetector sensitivityThreshold validation", () => {
+  it("rejects NaN instead of silently disabling score alerts", () => {
+    expect(() => new AnomalyDetector({ sensitivityThreshold: Number.NaN })).toThrow(
+      RangeError
+    );
+  });
+
+  it("still accepts finite values at the upper boundary", () => {
+    expect(() => new AnomalyDetector({ sensitivityThreshold: 1 })).not.toThrow();
+  });
+});