diff --git a/src/sdk/v4/_fetcher.ts b/src/sdk/v4/_fetcher.ts
index 25d9cb1..0fceb8b 100644
--- a/src/sdk/v4/_fetcher.ts
+++ b/src/sdk/v4/_fetcher.ts
@@ -33,12 +33,14 @@ export const fetcher = async <T extends object>(
     return camelizeKeys(json) as T;
   }
 
-  if (typeof fetch === 'undefined') {
-    throw new Error('fetch is not defined');
+  if (typeof globalThis.fetch !== 'function') {
+    throw new Error(
+      'Looks like there is no global fetch function. Take a look at https://quranjs.com/techniques#custom-fetcher for more info.'
+    );
   }
 
   // if there is no fetchFn, we use the global fetch
-  const res = await fetch(makeUrl(url, params));
+  const res = await globalThis.fetch(makeUrl(url, params));
 
   if (!res.ok || res.status >= 400) {
     throw new Error(`${res.status} ${res.statusText}`);
@@ -57,12 +59,14 @@ export const mergeApiOptions = (
   options: MergeApiOptionsObject = {},
   defaultOptions: Record<string, unknown> = {}
 ) => {
+  const clonedOptions = { ...options };
+
   // we can set it to undefined because `makeUrl` will filter it out
-  if (options.fetchFn) options.fetchFn = undefined;
+  if (clonedOptions.fetchFn) clonedOptions.fetchFn = undefined;
 
   const final: Record<string, unknown> = {
     ...defaultOptions,
-    ...options,
+    ...clonedOptions,
   };
 
   if (final.fields) {
diff --git a/test/fetchFn.test.ts b/test/fetchFn.test.ts
new file mode 100644
index 0000000..b57ba1b
--- /dev/null
+++ b/test/fetchFn.test.ts
@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import fetch from 'cross-fetch';
+import { quran } from '../src';
+
+describe('Custom fetcher', () => {
+  it('should fail with no fetch', () => {
+    // @ts-expect-error - we are testing this
+    globalThis.fetch = undefined;
+
+    expect(() => quran.v4.chapters.findAll()).rejects.toThrowError(
+      /global fetch/
+    );
+  });
+
+  it('should not fail if you pass a fetchFn', () => {
+    // @ts-expect-error - we are testing this
+    globalThis.fetch = undefined;
+
+    expect(
+      quran.v4.chapters.findById('1', {
+        fetchFn: (url) => {
+          return fetch(url).then((res) => res.json());
+        },
+      })
+    ).resolves.not.toThrow();
+  });
+});
diff --git a/test/setup.ts b/test/setup.ts
index 4abe764..c417650 100644
--- a/test/setup.ts
+++ b/test/setup.ts
@@ -1,13 +1,15 @@
 import fetch from 'cross-fetch';
-import { beforeAll, afterEach, afterAll } from 'vitest';
+import { beforeAll, afterEach, afterAll, beforeEach } from 'vitest';
 import { server } from '../mocks/server';
 
 // Establish API mocking before all tests.
 beforeAll(async () => {
   server.listen();
+});
 
+beforeEach(async () => {
   // we do this instead of passing a fetchFn every time
-  global.fetch = fetch;
+  globalThis.fetch = fetch;
 });
 
 // Reset any request handlers that we may add during the tests,