diff --git a/common/src/util/__tests__/lazy-response-ads.test.ts b/common/src/util/__tests__/lazy-response-ads.test.ts
new file mode 100644
index 0000000000..1b8e499f90
--- /dev/null
+++ b/common/src/util/__tests__/lazy-response-ads.test.ts
@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'bun:test'
+
+import { responseAdDisplayCount } from '../lazy-response-ads'
+
+describe('responseAdDisplayCount', () => {
+  it('returns eligibleCount when poolSize is at or above the max', () => {
+    expect(
+      responseAdDisplayCount({ eligibleCount: 5, poolSize: 100 }),
+    ).toBe(5)
+  })
+
+  it('clamps to poolSize when poolSize is below the max', () => {
+    expect(responseAdDisplayCount({ eligibleCount: 10, poolSize: 3 })).toBe(3)
+  })
+
+  it('returns 0 when eligibleCount is NaN', () => {
+    expect(responseAdDisplayCount({ eligibleCount: NaN, poolSize: 10 })).toBe(0)
+  })
+
+  it('returns 0 when poolSize is NaN', () => {
+    expect(responseAdDisplayCount({ eligibleCount: 5, poolSize: NaN })).toBe(0)
+  })
+
+  it('returns 0 when eligibleCount is Infinity', () => {
+    expect(
+      responseAdDisplayCount({ eligibleCount: Infinity, poolSize: 10 }),
+    ).toBe(0)
+  })
+
+  it('returns 0 when poolSize is Infinity', () => {
+    expect(
+      responseAdDisplayCount({ eligibleCount: 5, poolSize: Infinity }),
+    ).toBe(0)
+  })
+
+  it('returns 0 when both inputs are negative', () => {
+    expect(
+      responseAdDisplayCount({ eligibleCount: -5, poolSize: -10 }),
+    ).toBe(0)
+  })
+
+  it('floors fractional inputs', () => {
+    expect(
+      responseAdDisplayCount({ eligibleCount: 5.7, poolSize: 3.2 }),
+    ).toBe(3)
+  })
+})
diff --git a/common/src/util/lazy-response-ads.ts b/common/src/util/lazy-response-ads.ts
index 1b491a44d8..2214f85dea 100644
--- a/common/src/util/lazy-response-ads.ts
+++ b/common/src/util/lazy-response-ads.ts
@@ -16,8 +16,10 @@ export function responseAdDisplayCount(params: {
   eligibleCount: number
   poolSize: number
 }): number {
-  const eligibleCount = Math.max(0, Math.floor(params.eligibleCount))
-  const poolSize = Math.max(0, Math.floor(params.poolSize))
+  const safeEligibleCount = Number.isFinite(params.eligibleCount) ? params.eligibleCount : 0
+  const safePoolSize = Number.isFinite(params.poolSize) ? params.poolSize : 0
+  const eligibleCount = Math.max(0, Math.floor(safeEligibleCount))
+  const poolSize = Math.max(0, Math.floor(safePoolSize))
   return poolSize >= MAX_RESPONSE_AD_POOL_SIZE
     ? eligibleCount
     : Math.min(eligibleCount, poolSize)