diff --git a/src/builtin-filters.js b/src/builtin-filters.js
index 0217454..3f62be1 100644
--- a/src/builtin-filters.js
+++ b/src/builtin-filters.js
@@ -89,7 +89,7 @@ export function last(amount = 1) {
  * @returns escaped html
  */
 export function htmlentities(str) {
-  return str.replace(/\&/, '&amp;').replace(/</gm, '&lt;').replace(/>/gm, '&gt;');
+  return str?.replace(/\&/gm, '&amp;').replace(/</gm, '&lt;').replace(/>/gm, '&gt;');
 }
 
 /**
diff --git a/tests/builtin-filters.test.js b/tests/builtin-filters.test.js
index 7e574d5..16c3af0 100644
--- a/tests/builtin-filters.test.js
+++ b/tests/builtin-filters.test.js
@@ -3,67 +3,82 @@ import assert from 'node:assert/strict';
 import path from 'node:path'
 import { handleTemplateFile } from '../src/transforms/template-data.js';
 import { SissiConfig } from '../src/sissi-config.js';
+import { htmlentities } from '../src/builtin-filters.js';
 describe('builtin filters', () => {
 
   const dummyResolver = (map) => (...paths) => map.get(path.normalize(path.join(...paths)));
 
-  it('should format numbers', async () => {
-    const config = new SissiConfig();
+  describe('htmlentities', () => {
+    it('should not crash when nullish value is passed', () => {
+      assert.equal(htmlentities(undefined), undefined);
+    });
 
-    const vFS = new Map();
-    vFS.set('index.html', '{{ thousandPi | numberFormat: numberFormatOptions, "de-DE" }}');
-
-    config.resolve = dummyResolver(vFS);
-
-    const data = { thousandPi: Math.PI * 1e3, numberFormatOptions: {maximumFractionDigits: 2, minimumFractionDigits: 2} };
-
-    const result = await handleTemplateFile(config, data, 'index.html');
-
-    assert.equal(result.content, '3.141,59');
-  });
-
-  it('should format currencies', async () => {
-    const config = new SissiConfig();
-
-    const vFS = new Map();
-    vFS.set('index.html', '{{ million | currency: "eur", "de-DE" }}');
-
-    config.resolve = dummyResolver(vFS);
-
-    const data = { million: 1e6 };
-
-    const result = await handleTemplateFile(config, data, 'index.html');
-
-    assert.equal(result.content, '1.000.000,00 €');
+    it('should escape HTML', () => {
+      const expected = '&lt;h1&gt;Hello World&lt;/h1&gt;\n&lt;p&gt;Cow &amp; Son&lt;/p&gt;'
+      const result = htmlentities('<h1>Hello World</h1>\n<p>Cow & Son</p>');
+      assert.equal(result, expected);
+    });
   });
 
-  it('should format dates', async () => {
-    const config = new SissiConfig();
-
-    const vFS = new Map();
-    vFS.set('index.html', '{{ newYear | date: dateFormatOptions, "de-DE" }}');
-
-    config.resolve = dummyResolver(vFS);
-
-    const data = { newYear: new Date('2025-04-01'), dateFormatOptions: {"day": "2-digit", "month": "2-digit", "year": "numeric"} };
-
-    const result = await handleTemplateFile(config, data, 'index.html');
-
-    assert.equal(result.content, '01.04.2025');
-  });
-
-  it('should serialize json', async () => {
-    const config = new SissiConfig();
-
-    const vFS = new Map();
-    vFS.set('index.html', '{{ answer | json }}');
-
-    config.resolve = dummyResolver(vFS);
-
-    const data = { answer: {result: 42} };
-
-    const result = await handleTemplateFile(config, data, 'index.html');
-
-    assert.equal(result.content, '{"result":42}');
+  describe('integration with template engine', () => {
+    it('should format numbers via numberFormat filter', async () => {
+      const config = new SissiConfig();
+  
+      const vFS = new Map();
+      vFS.set('index.html', '{{ thousandPi | numberFormat: numberFormatOptions, "de-DE" }}');
+  
+      config.resolve = dummyResolver(vFS);
+  
+      const data = { thousandPi: Math.PI * 1e3, numberFormatOptions: {maximumFractionDigits: 2, minimumFractionDigits: 2} };
+  
+      const result = await handleTemplateFile(config, data, 'index.html');
+  
+      assert.equal(result.content, '3.141,59');
+    });
+  
+    it('should format currencies via currency filter', async () => {
+      const config = new SissiConfig();
+  
+      const vFS = new Map();
+      vFS.set('index.html', '{{ million | currency: "eur", "de-DE" }}');
+  
+      config.resolve = dummyResolver(vFS);
+  
+      const data = { million: 1e6 };
+  
+      const result = await handleTemplateFile(config, data, 'index.html');
+  
+      assert.equal(result.content, '1.000.000,00 €');
+    });
+  
+    it('should format dates via date filter', async () => {
+      const config = new SissiConfig();
+  
+      const vFS = new Map();
+      vFS.set('index.html', '{{ newYear | date: dateFormatOptions, "de-DE" }}');
+  
+      config.resolve = dummyResolver(vFS);
+  
+      const data = { newYear: new Date('2025-04-01'), dateFormatOptions: {"day": "2-digit", "month": "2-digit", "year": "numeric"} };
+  
+      const result = await handleTemplateFile(config, data, 'index.html');
+  
+      assert.equal(result.content, '01.04.2025');
+    });
+  
+    it('should serialize json', async () => {
+      const config = new SissiConfig();
+  
+      const vFS = new Map();
+      vFS.set('index.html', '{{ answer | json }}');
+  
+      config.resolve = dummyResolver(vFS);
+  
+      const data = { answer: {result: 42} };
+  
+      const result = await handleTemplateFile(config, data, 'index.html');
+  
+      assert.equal(result.content, '{"result":42}');
+    });  
   });
 });