diff --git a/src/logic/cache.ts b/src/logic/cache.ts
index cee17f5..d19d25c 100644
--- a/src/logic/cache.ts
+++ b/src/logic/cache.ts
@@ -22,7 +22,8 @@ export class Cache<T = any> {
   private ttlMs: number;
 
   constructor(ttlSeconds?: number) {
-    this.ttlMs = (ttlSeconds ?? Number(process.env.CACHE_TTL_SECONDS) ?? 300) * 1000;
+    const envTtl = Number(process.env.CACHE_TTL_SECONDS);
+    this.ttlMs = (ttlSeconds ?? (Number.isFinite(envTtl) ? envTtl : 300)) * 1000;
   }
 
   /**
diff --git a/src/logic/synth.ts b/src/logic/synth.ts
index 63e006f..d8e9b14 100644
--- a/src/logic/synth.ts
+++ b/src/logic/synth.ts
@@ -106,13 +106,14 @@ export async function synthesise(
   const choice = body.choices?.[0];
   const usage = body.usage ?? {};
 
+  const answerText = choice?.message?.content;
+  if (!answerText) {
+    throw new Error("OpenAI returned an empty response");
+  }
   return {
-    answer: choice?.message?.content ?? "No answer generated.",
+    answer: answerText,
     confidence: scoreConfidence(sources),
-    tokens: {
-      in: usage.prompt_tokens ?? 0,
-      out: usage.completion_tokens ?? 0,
-    },
+    tokens: { in: usage.prompt_tokens ?? 0, out: usage.completion_tokens ?? 0 },
     model,
   };
 }
diff --git a/tests/logic/cache.test.ts b/tests/logic/cache.test.ts
index d598a86..e36aa35 100644
--- a/tests/logic/cache.test.ts
+++ b/tests/logic/cache.test.ts
@@ -58,6 +58,25 @@ describe("Cache", () => {
   test("hitRate is 0 when no operations", () => {
     expect(cache.stats().hitRate).toBe(0);
   });
+
+  test("defaults to 300s TTL when no args and env var unset", () => {
+    const saved = process.env.CACHE_TTL_SECONDS;
+    delete process.env.CACHE_TTL_SECONDS;
+    const c = new Cache<string>();
+    c.set("k", "v");
+    // Should be retrievable immediately (not expired)
+    expect(c.get("k")).not.toBeNull();
+    expect(c.stats().size).toBe(1);
+    if (saved !== undefined) process.env.CACHE_TTL_SECONDS = saved;
+  });
+
+  test("respects CACHE_TTL_SECONDS env var", () => {
+    process.env.CACHE_TTL_SECONDS = "1";
+    const c = new Cache<string>();
+    c.set("k", "v");
+    expect(c.get("k")).not.toBeNull();
+    delete process.env.CACHE_TTL_SECONDS;
+  });
 });
 
 describe("Cache.normalizeKey", () => {
diff --git a/tests/logic/synth.test.ts b/tests/logic/synth.test.ts
index da3edbf..37e2bcd 100644
--- a/tests/logic/synth.test.ts
+++ b/tests/logic/synth.test.ts
@@ -76,4 +76,10 @@ describe("scoreConfidence", () => {
 
     expect(scoreAgree).toBeGreaterThan(scoreDisagree);
   });
+
+  test("single source scores lower than multiple sources", () => {
+    const one = [makeSource()];
+    const three = [makeSource(), makeSource(), makeSource()];
+    expect(scoreConfidence(three)).toBeGreaterThan(scoreConfidence(one));
+  });
 });