diff --git a/e2e/wdio/headless/test.e2e.ts b/e2e/wdio/headless/test.e2e.ts
index 1606a4fb4..3f54d095f 100644
--- a/e2e/wdio/headless/test.e2e.ts
+++ b/e2e/wdio/headless/test.e2e.ts
@@ -430,6 +430,25 @@ describe('main suite 1', () => {
             expect(await browser.execute(() => Math.random())).not.toBe(42)
         })
 
+        it('should escape the init script', async () => {
+            type WindowWithHello = Window & {
+                sayHello?: (name: string) => string;
+            }
+            const script = await browser.addInitScript(() => {
+                (window as WindowWithHello).sayHello = (name) =>
+                    `Hello ${name}`
+            })
+
+            await browser.url('https://webdriver.io')
+            expect(
+                await browser.execute(() =>
+                    (window as WindowWithHello).sayHello!('there'),
+                ),
+            ).toBe('Hello there')
+
+            await script.remove()
+        })
+
         it('passed on callback function', async () => {
             const script = await browser.addInitScript((num, str, bool, emit) => {
                 setTimeout(() => emit(JSON.stringify([num, str, bool])), 500)
diff --git a/packages/webdriverio/src/commands/browser/addInitScript.ts b/packages/webdriverio/src/commands/browser/addInitScript.ts
index 907fd495e..cc811a0ee 100644
--- a/packages/webdriverio/src/commands/browser/addInitScript.ts
+++ b/packages/webdriverio/src/commands/browser/addInitScript.ts
@@ -113,8 +113,9 @@ export async function addInitScript<Payload, Arg1, Arg2, Arg3, Arg4, Arg5> (
 
     const serializedParameters = (args || []).map((arg: unknown) => JSON.stringify(arg))
     const context = await this.getWindowHandle()
+    const src = 'return ' + script.toString()
     const fn = `(emit) => {
-        const closure = new Function(\`return ${script.toString()}\`)
+        const closure = new Function(${JSON.stringify(src)})
         return closure()(${serializedParameters.length ? `${serializedParameters.join(', ')}, emit` : 'emit'})
     }`
     const channel = btoa(fn.toString())