diff --git a/.changeset/bind-default-fetch.md b/.changeset/bind-default-fetch.md
new file mode 100644
index 0000000..36cc9de
--- /dev/null
+++ b/.changeset/bind-default-fetch.md
@@ -0,0 +1,5 @@
+---
+'@passmint/node': patch
+---
+
+Bind the default `fetch` to `globalThis` so the SDK works on Cloudflare Workers. The client stored the bare `globalThis.fetch` and invoked it as a method (`this.fetchImpl(...)`); workerd requires `fetch` to be called with its own global as the receiver, so every request threw `TypeError: Illegal invocation` unless a custom `fetch` was passed in options. Node's undici is not receiver-sensitive, which is why tests never caught it.
diff --git a/src/client.ts b/src/client.ts
index 39dea64..53ebc54 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -82,7 +82,12 @@ export class PassmintHttpClient {
     this.baseUrl = options.baseUrl ?? DEFAULT_BASE_URL
     this.timeoutMs = options.timeoutMs ?? DEFAULT_TIMEOUT_MS
     this.maxRetries = options.maxRetries ?? DEFAULT_MAX_RETRIES
-    this.fetchImpl = options.fetch ?? globalThis.fetch
+    // Bind the default fetch to its own global: we invoke it as
+    // `this.fetchImpl(...)`, and receiver-sensitive runtimes (Cloudflare's
+    // workerd) throw "TypeError: Illegal invocation" when fetch is called
+    // with any other `this`. Node's undici doesn't care, so only Workers
+    // users ever saw it.
+    this.fetchImpl = options.fetch ?? globalThis.fetch?.bind(globalThis)
     if (!this.fetchImpl) {
       throw new PassmintError(
         'global fetch is not available; pass a fetch implementation in options.fetch',
diff --git a/test/client.test.ts b/test/client.test.ts
index af003d7..21d577a 100644
--- a/test/client.test.ts
+++ b/test/client.test.ts
@@ -54,6 +54,33 @@ describe('PassmintHttpClient constructor', () => {
   })
 })
 
+describe('default fetch binding', () => {
+  const originalFetch = globalThis.fetch
+
+  afterEach(() => {
+    globalThis.fetch = originalFetch
+  })
+
+  it('invokes the default global fetch without a foreign receiver (workerd compatibility)', async () => {
+    // Cloudflare's workerd throws "Illegal invocation" when fetch is called
+    // with anything other than its own global as `this`. Node's undici does
+    // not care, so reproduce workerd's strictness in the stub — if the
+    // client ever stores the default fetch unbound again, this test fails.
+    globalThis.fetch = async function (this: unknown, ...args: Parameters<typeof fetch>) {
+      void args
+      if (this !== undefined && this !== globalThis) {
+        throw new TypeError('Illegal invocation')
+      }
+      return new Response(JSON.stringify({ ok: true }), { status: 200 })
+    } as typeof fetch
+
+    const client = new PassmintHttpClient({ apiKey: 'pmk_test_1', maxRetries: 0 })
+    await expect(client.request({ method: 'GET', path: '/v1/me' })).resolves.toEqual({
+      ok: true,
+    })
+  })
+})
+
 describe('VERSION', () => {
   it('matches package.json (regenerate with: node scripts/sync-version.mjs)', () => {
     const pkg = JSON.parse(readFileSync(new URL('../package.json', import.meta.url), 'utf8'))