diff --git a/frontend/src/lib/utils.test.ts b/frontend/src/lib/utils.test.ts
index 3fe64fb..7906be6 100644
--- a/frontend/src/lib/utils.test.ts
+++ b/frontend/src/lib/utils.test.ts
@@ -1,11 +1,22 @@
 import { describe, expect, it } from "vitest";
-import { cn, formatScore, statusBadge } from "@/lib/utils";
+import { cn, formatNumber, formatScore, statusBadge } from "@/lib/utils";
 
 describe("utils", () => {
   it("merges class names", () => {
     expect(cn("px-2", "px-4")).toContain("px-4");
   });
 
+  it("formats finite numbers compactly and handles invalid values", () => {
+    expect(formatNumber(1200)).toBe("1.2K");
+    expect(formatNumber(0)).toBe("0");
+    expect(formatNumber(-1200)).toBe("-1.2K");
+    expect(formatNumber(null)).toBe("—");
+    expect(formatNumber(undefined)).toBe("—");
+    expect(formatNumber(Number.NaN)).toBe("—");
+    expect(formatNumber(Number.POSITIVE_INFINITY)).toBe("—");
+    expect(formatNumber(Number.NEGATIVE_INFINITY)).toBe("—");
+  });
+
   it("formats scores", () => {
     expect(formatScore(87.4)).toBe("87");
     expect(formatScore(null)).toBe("—");
diff --git a/frontend/src/lib/utils.ts b/frontend/src/lib/utils.ts
index 15f08ef..f077955 100644
--- a/frontend/src/lib/utils.ts
+++ b/frontend/src/lib/utils.ts
@@ -6,7 +6,7 @@ export function cn(...inputs: ClassValue[]) {
 }
 
 export function formatNumber(n: number | null | undefined): string {
-  if (n == null) return "—";
+  if (n == null || !Number.isFinite(n)) return "—";
   return new Intl.NumberFormat("en-US", { notation: "compact" }).format(n);
 }