diff --git a/.prettierrc.js b/.prettierrc.js
index 80646ff..785bcee 100644
--- a/.prettierrc.js
+++ b/.prettierrc.js
@@ -1,6 +1,6 @@
 // https://github.com/ArthurFiorette/prettier-config
 
 module.exports = require('@arthurfiorette/prettier-config')({
-  tsdoc: true,
-  plugins: ['prettier-plugin-organize-imports', 'prettier-plugin-jsdoc']
+  tsdoc: true
+  //plugins: ['prettier-plugin-organize-imports', 'prettier-plugin-jsdoc']
 });
diff --git a/src/header/interpreter.ts b/src/header/interpreter.ts
index 5000592..f90c214 100644
--- a/src/header/interpreter.ts
+++ b/src/header/interpreter.ts
@@ -47,7 +47,7 @@ const interpretCacheControl: HeaderInterpreter = (cacheControl, headers) => {
       return maxAge * 1000;
     }
 
-    return maxAge * 1000 - Number(age) * 1000;
+    return (maxAge - Number(age)) * 1000;
   }
 
   return undefined;
diff --git a/test/header/interpreter.test.ts b/test/header/interpreter.test.ts
index 3cefc2c..07acbea 100644
--- a/test/header/interpreter.test.ts
+++ b/test/header/interpreter.test.ts
@@ -3,11 +3,13 @@ import { Header } from '../../src/util/headers';
 
 describe('tests header interpreter', () => {
   it('tests without cache-control header', () => {
-    const noHeader = defaultHeaderInterpreter();
-    expect(noHeader).toBeUndefined();
+    expect(defaultHeaderInterpreter()).toBeUndefined();
 
-    const emptyHeader = defaultHeaderInterpreter({ [Header.CacheControl]: '' });
-    expect(emptyHeader).toBeUndefined();
+    expect(defaultHeaderInterpreter({})).toBeUndefined();
+
+    expect(defaultHeaderInterpreter({ [Header.CacheControl]: '' })).toBeUndefined();
+
+    expect(defaultHeaderInterpreter({ ['x-random-header']: '' })).toBeUndefined();
   });
 
   it('tests with cache preventing headers', () => {