diff --git a/app/usage.test.js b/app/usage.test.js
index e0785b1..aa79008 100644
--- a/app/usage.test.js
+++ b/app/usage.test.js
@@ -244,4 +244,13 @@ describe('formatUsageStatsLine', () => {
     const line = formatUsageStatsLine('ollama', 'llama3', usage, { available: false, reason: '本地服務，無帳號額度概念' }, { hasData: false });
     assert.match(line, /；剩餘可用: 無法計算（本地服務，無帳號額度概念）/);
   });
+
+  it('produces safe text (no NaN) when quota/rate carry invalid numbers', () => {
+    // quota.limit 為 NaN、rate.limit 為 NaN → 不應算出百分比、不得輸出 NaN
+    const line = formatUsageStatsLine('openai', 'm', usage,
+      { available: true, used: 5, limit: NaN, currency: 'USD' },
+      { hasData: true, remaining: NaN, limit: NaN, kind: 'tokens' });
+    assert.doesNotMatch(line, /NaN/);
+    assert.match(line, /剩餘可用: 無法計算/);
+  });
 });