diff --git a/src/header/interpreter.ts b/src/header/interpreter.ts
index f90c2141..423698df 100644
--- a/src/header/interpreter.ts
+++ b/src/header/interpreter.ts
@@ -1,53 +1,49 @@
 import { parse } from 'cache-parser';
 import { Header } from '../util/headers';
-import type { HeaderInterpreter, HeadersInterpreter } from './types';
+import type { HeadersInterpreter } from './types';
 
-export const defaultHeaderInterpreter: HeadersInterpreter = (headers = {}) => {
-  if (Header.CacheControl in headers) {
-    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-    return interpretCacheControl(headers[Header.CacheControl]!, headers);
-  }
-
-  if (Header.Expires in headers) {
-    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-    return interpretExpires(headers[Header.Expires]!, headers);
-  }
+export const defaultHeaderInterpreter: HeadersInterpreter = (headers) => {
+  if (!headers) return undefined;
 
-  return undefined;
-};
+  const cacheControl = headers[Header.CacheControl];
 
-const interpretExpires: HeaderInterpreter = (expires) => {
-  const milliseconds = Date.parse(expires) - Date.now();
-  return milliseconds >= 0 ? milliseconds : false;
-};
+  if (cacheControl) {
+    const { noCache, noStore, mustRevalidate, maxAge, immutable } = parse(cacheControl);
 
-const interpretCacheControl: HeaderInterpreter = (cacheControl, headers) => {
-  const { noCache, noStore, mustRevalidate, maxAge, immutable } = parse(cacheControl);
+    // Header told that this response should not be cached.
+    if (noCache || noStore) {
+      return false;
+    }
 
-  // Header told that this response should not be cached.
-  if (noCache || noStore) {
-    return false;
-  }
+    if (immutable) {
+      // 1 year is sufficient, as Infinity may cause more problems.
+      // It might not be the best way, but a year is better than none.
+      return 1000 * 60 * 60 * 24 * 365;
+    }
 
-  if (immutable) {
-    // 1 year is sufficient, as Infinity may cause more problems.
-    // It might not be the best way, but a year is better than none.
-    return 1000 * 60 * 60 * 24 * 365;
-  }
+    // Already out of date, for cache can be saved, but must be requested again
+    if (mustRevalidate) {
+      return 0;
+    }
 
-  // Already out of date, for cache can be saved, but must be requested again
-  if (mustRevalidate) {
-    return 0;
-  }
+    if (maxAge) {
+      const age = headers[Header.Age];
 
-  if (maxAge) {
-    const age = headers[Header.Age];
+      if (!age) {
+        return maxAge * 1000;
+      }
 
-    if (!age) {
-      return maxAge * 1000;
+      return (maxAge - Number(age)) * 1000;
     }
 
-    return (maxAge - Number(age)) * 1000;
+    return undefined;
+  }
+
+  const expires = headers[Header.Expires];
+
+  if (expires) {
+    const milliseconds = Date.parse(expires) - Date.now();
+    return milliseconds >= 0 ? milliseconds : false;
   }
 
   return undefined;